I have a good tip for anyone who gets false alarms from PIR's.
I had it for years, and tried everything! It turned out that spideers would get into the hole at the back where the wires entered, especially the one in my living room that goes upstairs into the floorbaords, spiders would walk down the wire and into the back of the sensor.
I blocked the hole with blue tac, no problems since.
Tony, March 2007