Inaccurate Low Heart Rate Notification

I have two notifications on my watch for Low Heart Rate that “fell below 40 BPM for 10 minutes” one at 8:18 AM , the next one was 8:28 AM. Both giving the same reading. I was not wearing my watch, hadn’t even put it on for the day yet . It was charging.

why would it do this? Should I disable the nightstand mode?

I should mention I keep it on my nightstand to charge overnight right beside the base of my .homepod (it wasn’t in use)

I’ve since just restarted it.

Series 2 iOS 5.2.1