The Case Stained With Fluorescent Paint

The next day at nine, Seojin set the case marked with fluorescent paint on the white cloth in the analysis room. The case was old, and its handle bore scratch marks from fingernails. More noticeable than those marks was the blue powder caught inside the wheel. When she'd first seen the scene, she'd written that the powder had fallen from the stairway railing. It took the first hour of the morning to admit that sentence might be wrong.

The analyst turned the microscope screen toward Seojin.

"It looks like the same blue, but the resin is different. The lower layer is freshly applied paint, and the particles pressed on top are old powder."

"Could they have mixed during collection or transport?"

"They could have. That's why I won't fix the order of movement based on this result alone."

Seojin nodded. She covered the confirmed column on the result sheet and opened the alternatives column instead. The possibility that the collection bag had been jostled. The possibility that the case had passed through a different floor on the day of the incident. The possibility that someone had painted over it fresh at the scene. She wrote down all three and noted beside each what result would prove it wrong.

Minwoo peered at the paper from behind her.

"If you write it this weak, I can't tell what the report is even saying."

"It's not weak. It's so you can see where it went wrong later, if it does."

The analyst held out a second chart marked with a blind number. The blood protein wasn't under the fresh paint but caught between the particles on top of it. That meant it was a trace left after the case had made contact with the victim. But the fresh paint had already been pressed down before that.

Seojin pulled up photos from the day before the incident. The wheel already had a blue line on it, but the line in the photo taken at the stairway scene ran deeper toward the inside. It could mean someone had used the same route twice. Once to leave the paint, and once to move a person.

"The test run and the actual transport are separate events."

When she said this, Minwoo moved to write down Baek Sangho's name right away. Seojin pressed down on the tip of his pen.

"He's not the one who carried it out, not yet. That the route was used beforehand and that this person used it are two different statements."

Minwoo set his pen down. The blank space on the chart made the case feel heavier, not lighter.

At eleven, Gyeongmin brought a backup device from the maintenance room. It was an old counter, not connected to the main server. It only kept a record of how many times the emergency key had been turned, nothing about the exact time or who had operated it. Gyeongmin set the case down and checked his own signature first.

"I managed this device. Any missing records are my responsibility too."

"Then that's one more reason not to withhold it."

Gyeongmin looked up at Seojin's words.

"If I hide it, my negligence won't show up, but neither will any record of Baek Sangho accessing it. Let's make two copies."

Minwoo photographed the numbers on the counter and cross checked them against the motor vibration log. On the afternoon before the incident, there was a stretch where the number increased twice. The vendor's work schedule had nothing listed for that time. Baek Sangho was the only one who had entered during that window. The entry log alone wasn't enough, but the emergency key test that wasn't on the official schedule and the fresh paint had landed on the same day.

Gyeongmin wrote "unlogged test confirmed" in the blank line of the work schedule. When Minwoo asked why he was writing his own name into it, Gyeongmin pointed to the seal on the back of the device.

"A missing record isn't only the fault of whoever left it out. I was the one managing it, and no one checked."

After that, Seojin printed out her own earlier sentence, the one where she'd underestimated the fluorescent powder. Minwoo looked at the paper without a word. Below the sentence, Seojin wrote her reason for the correction. "The order of the fresh paint's prior compression and the blood contact has been confirmed, so the hypothesis of accidental post incident use is withdrawn."

At the afternoon meeting, Yeoreum broke the outside editor's role into three lines. Did he receive the file in advance. Did he know about the plan before the harm occurred. Did he carry out the edit after the harm occurred. Jang's payment and logs pointed to the third line, but also touched the first. The second line was still empty.

"We can't fill that blank with Baek Sangho's statement," Yeoreum said. "Just because he's lying doesn't automatically mean someone else's awareness grows to match."

Seojin found the phrase "security test" in Jang's message. The phrase matched one Baek Sangho used in internal company documents, but it wasn't confirmed whether Jang had received that exact sentence or chosen it himself. The investigation team asked for the transmission route instead of a conclusion.

Just then Dabin's phone rang. There was no caller name on the screen. She took a breath and pressed the call button.

"I'll give the USB back." It was Baek Sangho's voice. "In exchange, tell me the name of the child you're hiding."

Dabin's fingers went rigid. When Gyeongmin reached for the call recording button, she raised her palm to stop him. Seojin watched Dabin's expression and thought of her own ability. If she heard just one word, she felt she could confirm what Baek Sangho was hiding. But that word always left a cost behind, and she still hadn't decided who would bear it.

"Send proof first that the USB is really in your hands," Dabin said. "A person's name isn't something you trade."

Ten minutes after the call ended, a photo arrived. A locker number and a sealed black case. In the corner of the photo sat a receipt stamped with a blue triangle. Seojin zoomed in on the receipt and stopped. Part of the customer number overlapped with an earlier payment record.

Minwoo said,

"If we go now, we can open the locker."

"Don't open it. Preserve the location," Dabin answered. "I'm going to call in the agency I've chosen, together with you."

Gyeongmin raised no objection. If Gray Thread monopolized the evidence, Baek Sangho could try to change who held the upper hand in the deal. Dabin contacted a legal agency and requested a joint sealing procedure, and Minwoo secured the footage from the camera in front of the locker.

Seojin wrote a final sentence on the analysis chart.

"The test run from the day before the incident has been confirmed. The person who carried it out narrows to Baek Sangho, but the motive and his exact role still need separate verification."

At that moment, a failed remote access record appeared on the screen of Gyeongmin's backup device. The locker's power had flickered twice, and the account behind the access attempt was registered to Baek Sangho. Dabin got her first chance to seal the case without ever having seen the USB. Seojin left the ability use column blank and moved the disproof chart to the next line. An attendant was already waiting in front of the unmanned locker. Gyeongmin didn't ask for the management code, and instead set his phone up where the camera's view and the time stamp would both show. When Minwoo took out the sealing tape, Dabin spoke first.

"I'll check the tape myself. If we put an investigation team mark on it before we know what's inside, that mark itself could look like pressure later on."

Minwoo held the tape for a moment, then handed it to her. Dabin read the seal number aloud, then had the legal agency staff read the same number back. Only after the two numbers matched did Gyeongmin cut power to the locker. The door didn't open. All that appeared on the screen was the message "remote command denied."

Seojin photographed the screen and wrote down Baek Sangho's name and the time separately. That alone didn't mean the access attempt was a bid to seize the USB. The account could have been hijacked, or the person who sent the photo might not be the real owner. She left that possibility on the chart. This time, Minwoo said nothing.

The attendant checked the size of the sealed case. It matched the photo, but there was no telling what was inside until it was opened. Dabin didn't reach for the case. Instead she wrote "informant identity withheld" on the joint custody confirmation form.

Seojin looked at that sentence and asked,

"What if that request isn't accepted?"

"Then we'll decide the scope of disclosure again at that point. Right now we're only deciding what we can decide."

Dabin's voice shook, but her sentences were long. Instead of bouncing the question back the way she had the first time they met, she was setting her own terms first. Seojin didn't note the change. Someone's growth wasn't a title an observer attached, it had to show up in the next action.

That evening, back in the analysis room, Seojin looked again at the cure hardness of the paint sample. The direction the fresh paint had been pressed the day before the incident didn't match the pressure of the wheel going down the stairs. Once the wheel had been pushed slowly, and once it had taken a sudden load. It was possible the same person had used the same equipment both times, but that too was still a hypothesis.

Seojin didn't write "for testing purposes" on Baek Sangho's work schedule. Instead she wrote "two uses of the emergency key not on the official schedule." The blank space remained, but its shape had changed. Before, it had been time no one had seen. Now it was an action someone had failed to log.

Minwoo asked from the doorway,

"Still, if it wasn't Baek Sangho, who would go this far?"

"That question is about motive. What we need right now is a list of who could have done it."

"Baek Sangho will be the only one left on that list."

"Then write down that result when it happens. Don't write the answer you're expecting before you get there."

Minwoo smiled, looking tired. "You really do manage to annoy people to the end."

Seojin almost smiled but pressed her lips down instead. The word loss column in her notebook was empty again today.

At nine that night, Jang's additional logs arrived. The file transfer hadn't happened only once, after the harm occurred, it had also happened the night before the incident. The transfer was titled "corrected version for field of view test." She hadn't opened the file's contents, but she could confirm its hash and the transfer time. Baek Sangho's account had downloaded that file once, and Jang's account had logged into an outside editing service twelve minutes later.

Seojin didn't jump to a conclusion even after seeing that record. She split it into separate questions: who had known what content, whether the file name matched its actual contents, and whether Jang had recognized the corrected version as a murder plan. Yeoreum read through that list of questions and decided to call Baek Sangho in again.

A new message had arrived on Dabin's phone.

"Next time, it won't be the child's name. I'll expose the reason you're hiding instead."

Dabin didn't delete the message. She put it into the joint sealed folder instead. This time she asked Gyeongmin first whether the original had been preserved. When he nodded, Dabin photographed the screen and personally signed to confirm that she had kept that message.

Seojin attached the customer number from the blue triangle receipt inside the case file. The digits alone couldn't confirm anything. But the two layers of paint on the case, the two uses of the emergency key, the file transfer from the night before the incident, and the failed remote access to the locker all tied together into a single question.

"What did Baek Sangho prepare, what didn't he know, and what did he hand over to whom."

Seojin wrote down that question, then closed her pen. The answer hadn't come yet. Instead, the locker's seal number and the sample number on the analysis chart went into the same case file. That much was enough evidence to start the next day with.