Doesn't look like any co detector I have seen. Usually a co or smoke detector would be heavily vented to allow smoke/co to enter. There isn't much venting at all on that.
That looks like some kind of ir or motion detector.
There should be some kind of spec written on it somewhere if it's a smoke or co... maybe on the back. At least a date because by law they need to be changed out every 10 years or something like that.