Heart rate monitored when I'm not wearing the watch

I am getting notifications of "elevated heart rate during a period of inactivity" and I have seen the green sensor lights - all when I am not wearing my watch.

This started happening after I updated to watchOS 4.1. I started receiving notifications that my heart rate was elevated during a period of inactivity - but the log shows the rates were detected during the hours I wasn't wearing my watch. I shrugged it off as a fluke at first, but I continue to get the notifications. The heart rates detected range from as low as 120 bpm to as high as 176 bpm. I've also seen the green sensor lights flashing when my watch was sitting on my table.

What's going on and how can I stop this from draining my battery and mucking up my heart rate monitor log with bunch of false readings?