False motion alarm last night - Unexplained

[briansgooglegroupemail@xxxxxxxxx]

So I had my first false alarm last night, two years after having
installed my Ademco system.  It was a motion detector in my garage that
tripped, which was curious since the side garage door sensor had not
been tripped.  Called the cops in anyhow, and as I suspected, the coast
was clear.  So I went out to inspect the sensor... One thing I noticed
was that there were ALOT of cob webs around the motion detector that
tripped.  I cleaned them all away, but what I'm wondering is what can
it really take to trip one of the wireless Ademco motions?  I haven't
found any trace (droppings, etc) of any varmits in the garage, so am
rather stumped.  I would love to hear theories or just "once in 2 years
aint bad" from the experts.

Thanks!!