The Locked Office

Laura Chen was always the first person to arrive at the accounting firm where she worked, on the ninth floor of an old brick building near Boston Harbor. On Monday morning, at a quarter to eight, she unlocked the door of her small office, switched on the light and stopped. Her computer was on.

That in itself was not impossible. Laura was sure she had shut it down on Friday evening, but she had been tired, and she had left in a rush to catch her train. Then she noticed the plant. It was a sad little fern that she kept on the windowsill and regularly forgot about. On Friday, its leaves had been brown at the edges and its soil had been as dry as sand. Now the soil was dark and wet, and there was a small puddle on the saucer underneath it.

"Somebody's been in here," she said out loud, just as her colleague Daniel appeared in the doorway with two cups of coffee. Daniel listened to her story with growing interest. He read a lot of detective novels and had always wanted a real mystery. "Okay," he said, sitting down. "Let's think about this logically. Who has a key?"

Only three people had a key to Laura's office: Laura herself, the office manager and the building's security staff. "It can't have been the office manager," Laura said. "She's been on vacation in Florida for a week." Daniel nodded. "And it can't have been the cleaners. They don't come on weekends, and they never touch anything on people's desks anyway."

Laura sat down at her desk and noticed something else. Her chair was much lower than usual, so low that her chin was almost level with the keyboard. "Whoever it was must have been sitting here for quite a while," Daniel said. "Nobody adjusts a chair if they're just checking an email. And they must be shorter than you, which isn't easy."

Laura moved the mouse and the screen came to life. A window was open, showing a message: Updates are being installed. Do not turn off your computer. "That explains why it's still on," Daniel said. "They must have tried to shut it down, and then the updates started. They couldn't have switched it off without risking damage, so they just left it." Laura frowned. "So whoever it was was trying to be careful. That's not what a thief does."

Under the keyboard, Laura found the final clue: a yellow sticky note covered in small, neat handwriting. It said: Depreciation = cost minus residual value, divided by useful life. Ask someone about chapter 7. Laura stared at it. It was a basic accounting formula, the kind of thing students learn in their first year. "Someone must have been studying," she said slowly. "Someone who has a key, works here on weekends and is learning accounting."

They looked at each other and said the same name at the same time: "Frank." Frank was the security guard who worked the weekend shifts. He was in his fifties, short, cheerful and always reading something at the front desk. Laura remembered that a few weeks before, he had asked her, a little shyly, what exactly an accountant did all day.

At lunchtime, Laura went down to the lobby and found Frank finishing his shift. When she showed him the sticky note, he turned pale. "I'm so sorry," he said. "I should have asked first. My laptop broke last month, and I'm doing an online accounting course. The exam is in two weeks. I've been using your computer on Sunday mornings, when the building's empty. I thought nobody would notice." He paused. "I watered your plant to say thank you. It looked like it was dying."

Laura could have been angry, and Daniel was clearly expecting a dramatic scene. Instead, she smiled. "Chapter 7 is depreciation," she said. "It confuses everyone. If you're free on Thursday after work, I'll explain it to you properly." Frank passed his exam three weeks later with the highest grade in his class. He now works in the firm's finance department, two doors down from Laura, and her fern has never looked healthier.