Strange PIR behaviour

No - I've got several PIR's and some of them run off a wall plug which also has a kettle and toaster nearby and they dont set it off. Perhaps your wiring is struggling to maintain a good voltage when things are switching on and off so the PIR thinks it's being switched?

I wouldn't have thought it's noise.

Yes, it's very common. I've got one PIR controlling a front door light that merrily came on whenever any coventional fluorescent light in the house was turned on. Cured by fitting a noise filterin the feed to the PIR.

interruption,

Did you install it? check for loose connections inside the lights housing and any water getting into the housing, also the connection to the fuse?

Either that or it could be just coincidence ie spiders coming out from behind for a daily workout. ;-)

Interesting! Is there a standard filter I can buy, and how does it need to be connected - e.g. a capacitor across the mains, or across the PIR, or a choke in series with the lamp feed, or maybe some combination thereof?

Yes, I did install it - so it's very unlikely that the connections are loose. I think it's more than co-incidence, 'cos I can more or less make it do it to order.

I think that Peter Parry is probably on the right lines with his suggestion of a filter being needed.

In article , Roger Mills scribeth thus

Some of the earlier ones were very sensitive to radio energy and interference, more recent ones are better in that respect!..

The one I used was made by Ferranti, weighed about 10lbs and came from a Bloodhound missile system so may not be available in the shops. I'm pretty sure I've got a box of much smaller plug in ones in the garage in German (Shuko) plug/socket adapters but if you take them apart the PCB's fit into a standard switch box. They wire in parallel with the lamp, very simple, give me a ring or e-mail if you would like one.

I once installed an alarm system with some cheap PIRs. Radios in passing police cars and taxis would set it off The Band III PMR transceiver in my vehicle would trigger it when I was parked outside.