Electronic Alarms and Home Security Devices - False PIR trips

I have an installation consisitng of Ademco Vista-10P with 2 Bosch ISM BLP1 PIR sensors, 3 Bosch DS938Z 360° PIR sensors and 1 entry door magnet switch. This system was professionally installed in December 2008 and operated normally until Sunday, June 8 when we received a single event false alarm from one of the 360° PIRs at 4:05am. On Sunday, June 14 at 12:23am another false alarm from one of the ISM BLP1 PIRs, again a single event resetting after 4 min. The alarm company sent a technician who spent 2 hours testing the system, checking the wiring, connections and program and sensors. We went one week with no problem and then on Monday, June 29 at 4:42am another single even false alarm at a different 360° PIR. There are no heaters or A/Cs to introduce temperature changes and the building is in an industrial area with no activities at this time of morning on weekends. So far the alarm company is at a loss to explain the problem, suggesting possibly a weak battery during a system check however I have checked the battery and it has a normal 12-13 volts when the transformer is unplugged. Do you have any suggestions as to possible causes? Is it possible line noise could cause random PIRs to trip? None of the suggestions offered so far make sense, the alarm company will schedule another tech test but I don’t hold out too much hope.

The simple truth of the matter is that it is probably an environmental thing that you just haven't figured out the pattern to. I have seen motions set off by everything from silverfish crawling into the housing to laser printers ejecting sheets of hot paper.

Honestly, the odds are against a hardware failure, given the diagnostics you have had done.