Hi again. Thanks for your reply and confirming your system.
You are right to have the pir as an entry device as this means you can come down stairs and set the timer rather than setting off the alarm and also that it is still protecting whereas a pir set to 'home' in that scenario would be inneffective since you would then in effect only have a door sensor monitoring downstairs. The suggestion to move the sensor is the next logical step.
Just one suggestion - you say you changed the batteries - please do not tell me you put duracells in? They can be problematic. A generic brand is usually better there are issues with the fit of duracells in the holders.
That might possibly be an issue if the pir is in effect switching itself off and on. I don't see why it should be but hey you never know.
There does not appear to be any obvious reason from your picture. Is there anything at all that might be creating a heat signature that was not there before. If you have cross checked the pir's as you say it can really only be a sensor location issues.
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