Are my Monoxide detectors faulty?

Hi, tonight I was awoken by my uncle and he claimed the Monoxide detectors had been going off, he brought one up to where I live and sure enough it was around 213 and 800+ of levels. I'm not EXACTLY sure if this was a Monoxide detector but it kept blinking *GAS* *GAS* *GAS*. He plugged the one that detected it into our wall upstairs. I went downstairs and all windows and fans where open/on, no reports of dizziness except for mine(Up all night making sure everyones OK). We have a total of about 4 of those detectors. We plugged the one we brought upstairs into the wall downstairs and oddly... the same one beeped an hour later. But it read the level was 0 but *GAS* so we turned it off thinking it might be faulty and anyone who was in a room with the detector is still fine... the other 2 detectors didn't go off, however. ( 2 of them went off with a level of 0 and blinking *GAS*) but I'm wondering if there's a worry or if we got a faulty one.