Most smoke alarms operate by allowing the smoke to get between a light source and a light sensitive target. When the light gets dim, the alarm goes off. If a spider passes in front of the light, or spins his web in front of the light, he may just set off the alarm. Vacuum out your smoke alarms regularly.
By the way, if you want to test a smoke alarm, pushing the "test" button only tests the electronic circuits, not the light detection system. It is best to first test it with the button, then test it with smoke from a stick of incense. When you want to turn it off, just blow the smoke out by waving a newspaper at it -- often the fastest technique. Some new alarms have "hush" buttons to turn off that false alarm.
When you buy a new smoke alarm you will notice that it is only guaranteed for a certain number of years, 5, maybe 10 or 15. How long have yours been installed. We tend to fogret about them, especially if they are wired into the house and do not require changing the battery. I am happy to discover in 2012 that some are now putting stickers on the side of the alarm telling you when they should be replaced -- like the "best before" date on food products. If your's don't have expiry dates in evidence right on the side I recommend that you use a permenant marker and write, not the purchase date, but the replacement date. If you don't know, put that date 10 years after you think they were installed. You may be suprised to discover that they already need changing.
Smoke alarms do save lives, but only if they work.