I have an alarm system that has a PIR in the detached garage. Every time we go on holiday the alarm triggers via the garage PIR. I can pinpoint it to when the dustbin men come by looking at the alarm log, and also my neighbour tells me! I leave a key with her so she can turn off the alarm and reset.
Now my (limited) understanding of PIR's is they work on heat/movement. How is it being triggered by the bin lorry? The lorry only gets as close as approx 50 feet, so I do not think vibration is the cause. My thoughts are the flashing lights on top of the cab. There is a gap at the top of my garage door of approx 3/4"and the PIR is moulted at approx this height, is it possible that the 'lighthouse effect' of the lorry light is somehow crossing the PIR sensor and triggering the alarm or am I barking up the wrong tree.
I have had a guy look at it (not an alarm engineer but an electrician acquaintance of a friend) and he seems to think it will be down to spiders although he couldn't see any evidence of webs etc in or around the PIR, also the spider theory wouldn't explain why it only goes off on a Weds morning when the bin truck arrives, unless of course the spider is hiding from the bin men;-)
Cheers
John