Carbon Monoxide Detector Going Off But No Gas Appliances In Apartment?

Last night I was awoken by our carbon monoxide detector going off, so I opened the windows and unplugged it because I didn't want it to wake up all of the other people in the building if it went off again. I plugged it back in when I woke up and about two hours later it went off again. It is only going off at the warning level and not the alarm level, and we have no gas appliances in the apartment and our heat has not been turned on yet. Is the device malfunctioning or is it possible that something is leaking carbon monoxide?