Commercial alarm sensors have a tamper contact on the cover, put a microswitch contacting the table on the bottom of the box and out of sight, if someone lifts the box up the switch actuates and your elctronics detects it, in conjunction with bobs infra red self test your system will be well protected.
Another theft prevention and combined covering up protection would be to use a visible or infra red led and a retroflective strip stuck on a wall in the room, the electronics pulses said led and looks for a reflection, if it doesnt get it then its either covered up or has been moved, but this does mean sticking a retroflective strip somewhere in the room.