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"Who's there?"

Shen Qianqian muttered, puzzled.

She mostly lived alone and had few visitors aside from Xue Qiqi and Xia Qingqing, with male friends being even rarer – Lin Chuan was probably the first.

And most likely, the only one.

Father Shen was drunk, and Mother Shen should have been at home tending to him, so she was somewhat perplexed as to who would be knocking on her door at this hour.

— The reasoning of a professional detective.

Under the dim yellow light, Shen Qianqian stood up and walked toward the door, peering through the peephole, only to find an empty, pitch-dark space outside – she saw nothing.

No one?

Shen Qianqian was somewhat startled.

"Who is knocking?" Lin Chuan came out too, and several seconds had passed without Shen Qianqian opening the door, prompting his curiosity.

Shen Qianqian shook her head: "No one."

"No one?" Lin Chuan frowned.

"It might be some kids playing a prank," Shen Qianqian guessed.

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"Let me check."

Lin Chuan took Shen Qianqian's hand and pulled her behind him, then approached the door and peered through the peephole.

The next moment, his brow furrowed.

"What's wrong?" Shen Qianqian, seeing Lin Chuan's unusual reaction, asked softly.

Lin Chuan placed a finger on Shen Qianqian's lips, signaling her to be quiet. He then picked up his suit jacket from the shoecabinet, where he had placed it upon entering.

Shen Qianqian pursed her lips, confused.

Lin Chuan's left hand slowly reached for the doorknob, and then suddenly he flung the door open, while his right hand pressed the suit jacket in front of him.

Bang!

The door swung open with great force, creating a noise louder than usual, and the voice-activated light outside immediately lit up.

The door was open.

But there was nothing outside.

Lin Chuan stepped out and glanced around before turning back to the entrance, deep in thought.

"Is something wrong?"

Shen Qianqian, startled by Lin Chuan's abrupt actions, felt her heart rise with concern.

Lin Chuan smiled: "I was worried that someone might knock and then hide in a blind spot of the peephole, ready to attack as soon as we opened the door."

"You're really vigilant, but it was probably an overreaction," Shen Qianqian said with a relieved smile.

Lin Chuan pointed to the ceiling light and asked, "Was the light on when you looked out?"

"No, it wasn't," Shen Qianqian replied.

"The voice-activated light takes about 30 seconds to turn off. From hearing the knocking to opening the door, it took about 15 seconds, so if it were a prankster, you should have seen the light on through the peephole," Lin Chuan explained.

"Hmm?"

Shen Qianqian's face showed confusion.

With Lin Chuan pointing this out, she went from relaxed to vigilant again.

Lin Chuan then pointed to the elevator.

"What about the elevator?" With Lin Chuan beside her, she asked almost subconsciously.

This building had two elevators for every four units.

Currently, one elevator was stopped on the 1st floor, and another was ascending from the basement level.

"According to the typical elevator speed for this building, it takes about 25 seconds to travel from the 30th floor down to the 1st floor. When I stepped out, the status of the elevators was as it is now, so it ruled out the possibility that the person who knocked on the door took the elevator down. When we were at the door, we didn't hear the elevator open, nor did we hear any of the three neighboring units open their doors, so that person must have been in the stairwell."

Lin Chuan finished speaking and his gaze fell on the slightly ajar stairwell door.

It was very possible that the person was right behind that door or listening to everything from within the staircase.

Lin Chuan's first career experience had been as a thief.

So he knew these tactics inside out.

He guarded Shen Qianqian behind him and then suddenly kicked at the door.

Bang!

A harsh noise echoed through the air as the stairwell door was forcefully kicked open.

Inside, it was completely empty.

"A misjudgment?" Lin Chuan muttered, furrowing his brows as he stepped into the stairwell. The voice-activated light had already turned on, illuminating the stairs, floor by floor.

"How can there be no one?" Shen Qianqian also frowned, following behind Lin Chuan.

Lin Chuan crouched down, his gaze landing on the ground just behind the door to the stairs.

They were on one of the upper floors.

Even if there was a power outage, it was rare for anyone to come up here, especially to the corner behind the stairwell door. Such places were usually only made glossy in lurid comic books or illicit films.

"Did you find something?" Shen Qianqian inquired.

Lin Chuan pointed to the floor: "Look here, there are slight friction marks from shoe soles."

Shen Qianqian looked in the direction indicated, and on a thin layer of dust on the floor, there were two extremely fine marks, like the grind of a pestle, with some messy patches nearby.

It was clear someone had been here, squatting on the balls of their feet.

Shen Qianqian's heart tightened: "Where did he go?"

Lin Chuan didn't respond but instead examined the scene up and down the stairwell and on the rooftop.