I remember when I was at University someone managed to set off the fire alarm due to spraying hairspray or deoderant whilst standing under the sensor. Surprisingly that was the only time it went off during the whole year, beyond the one fire drill. The neighboring block though seemed to have theirs going off almost every week. Luckely we didn't have sensors in the kitchens, and there was a double door from the kitchen to the corridor (i.e one door, then a short passage, then another door), so smoke didn't get out. I remember one time looking into one of the other kitchens and seeing large flames coming from the top of the grill, before someone rushed in and delt with it. No idea what that was caused by.