(Thanks!) Hmm, that's really annoying, could it be a carbon monoxide alarm? Gotta say, a few of my housemates didn't react to the fire alarm until we all came running downstairs screaming "get out!", which is kinda frightening, some people are conditioned to think - "oh its probably a prank, or a drill, or a some other reason I shouldn't react". Hard to judge when the real thing is happening... Be safe!
Yeah, that's why we opened all the windows. One room is locked and might have someone sleeping in it - we can't tell, he won't answer our calls, but it isn't fire because we'd have noticed that by now. I really hope it is not something awful. Chance is slim, though. Our fire alarm goes off at least once a week because someone cooks terribly. Whenever it goes on for longer or more often than that, people do react, thankfully. It does help that our fire alarm is about as loud as an air horn - can't really be ignored. Which makes it all the more annoying that we're now entering hour 4 of fire alarm bonanza.