The janitor

His trick

The graveyard shift custodian has keys to every nook and cranny, and codes for every secure area. He's probably going to school and plans to work as an IT security specialist for your competitor, borrowing your books and data as material for his lab classes. He frequently finishes his rounds early, takes advantage of the network connection, uses your technical books as a lending library, dropping doughnut dust as he reads printed materials left out on every desk.

Your treat

Work with the facilities department to assign specific cleaning crews to specific areas, and require that they sign in and out. Tell them you're increasing security in the area, and paint a few smoke detectors shiny black so they look like smoked glass security camera bubbles at night. If the department isn't cooperative, find out where the custodian parks himself for study hour by asking who keeps finding their ergonomic chair frequently readjusted and network cable looped up onto the desk. After hours, leave a small pile of soiled bandages and rumpled printout on necrotizing fasciitis at that station, and see if he really wants to be caught lounging there again.