False PIR switching

[AdrianUK]

We live in a three storey house

The lighting for the top flight of stairs is done by a self contained PIR/lamp bulkhead type fitting, the kind that is often found outside, installed by the previous owner & I've never 'gotten round' to changing it to a more conventional arrangement.

This unit will often trigger at unpredictable times - sometimes during the day, sometimes in the middle of the night. I've always put this down to spikes/interference on the mains and a 'cheap' detection circuit. I guess the circuit kind of 'fails safe' ..... if in doubt - activate!

 

[RF Lighting]

I've had my burglar alarm go off in the middle of the night and found a large spider sat on the lens of it when investigating the cause, so spiders are not an impossibility.

 

[ban-all-sheds]

It is. However, some people clearly perceive a benefit in the less simple, less obvious (and less cheap) approach, even though it comes with some downsides!

Then they have a significant problem with perception.

As the OP demonstrates.