Can a spider set off a PIR intruder detector?

There's this charity office (in a small town in the UK home counties) in which I'm a volunteer, and every couple of months, in the wee small hours, the alarm goes off with no apparent cause. The control panel records which detector has been set off, and the maintenance contractor suggests that a spider walking across the detector can be enough to set it off. I suppose that the cleaner, working after the office has closed, will disturb spiders and perhaps make them wander around, but can they really do this? Hope you can help, please.