Is it what it is, or not?

   At 3:30 this morning, one of the 3 smoke/CO2 detectors started beeping, 3 loud beeps in succession. I can't remember what that means, whether fire or carbon dioxide,  so I turned the furnace way down, just in case. Detector is the 10 year sealed type, no batteries to replace. I'm sure it's not 10 years old.  I put it outside where it continues to beep. The cat is unhappy with the sound, but seems coherent. I did the online word puzzle, but could only come up with mostly 4-letter words. 

   It's now 5:15 and the beeping has stopped from the sensor's position on the porch rail. so I brought it back into the house. I'm going back to bed. If it beeps again, I guess I'll call the Fire Dept.  But wait, they have hardly any staff. Oh, well.