Health App reporting steps at night when the phone is not in use

There have been multiple instances when the Health App reports steps taken in the middle of the night when the phone is not in use. It has happened 3 times in the last 8 weeks. There is no pattern but the app reports steps were taken over the early morning hours while I was sleeping. In all three cases, I have left my phone that night on a charger in another room.

I have my phone locked, my passcode is secure (to the best of my knowledge). I do not see any Screen time that might indicate the phone was being used by someone. But then again, I don't think you need to unlock the phone to track steps.

Is my phone somehow being hacked? The Health app is linked to my Medical insurance app.

Has anyone else observed this behavior?