Please pardon this answer form another Noob:
Nah - a fly won't do it. The motion sensors detect heat, and specifically heat movement. I'm assuming this is a new setup? Where do you live? I've found the heat from my heating system can set off my sensors if they're in the path of the heating vents, and the heat from my home theater setup also has triggered a false alarm.
Try repositioning the sensor LOWER. Remember, it's field is out & down. If there's a possibility of warm-blooded critters on the floor, try adding 1/2" masking tape to the bottom of the sensor lens (solved my Yorkies' trippin the sensor problem).
Also, if you have AHP, read the activity monitor logs from when you're not in the room (and the alarm's not set) - look for triggers - .
Another suggestion found somewhere here was to but a single-thickness of waxed-paper over the lens - it's suppose to reduce the sensitivity?
Whatever you do, make sure you test with a human afterwards to be sure it triggers when you want it to trigger!