Carbon monoxide detector

[honest mark]

First time ever I heard a beep , couldn’t figure out what it was, the red light was on my carbon monoxide detector, could not shut it off?? Took cover off, there is no battery??? When got back to dock checked engine compartment every thing seemed fine??? It was 1 beep like every 10 or 20 seconds??? How do I shut it off??? Any thing to check or clean??? Thank you