Fishy moves at night

Today, due to some reasons I factory-reset my iPhone. When I’m setting up (pressing those “Get Started”), I found something fishy in the Health app. I rarely work after 1am, and recently I did start sleeping before midnight. I found that there are some walking records, timestamped at night, around 3-6am. My immediate guess is that someone moved my phone at night during my sleep. I’m not sure whether it is normal. For example, is iOS using location service to locate my phone’s approximate location so as to calculate how many “steps” I *walked*, or is it solely using the motion sensors? I usually put my phone near my pillow. However, because the numbers of steps are not small (at least 10), it should not be caused by my movement during sleep time. Since I’m not asking with respect to information security, I’m not looking for counter-measures. So any ideas about the causes are welcome, even if they don’t address to solutions.