A mate of mine has his shower extractor fan controlled by PIR at my suggestion, although despite being masked off for all areas other than the shower cubicle, it does occasionally trigger itself for no apparent reason. It's not repeatable enough to diagnose the fault, so might just be due to a cheap or dodgy PIR, draughts from the radiator, etc. I don't expect he paid much for it, so perhaps it's just a case of you get what you pay for, but it's still something to be aware of.
