Yeah I checked some tracks and it basically looked like the longer someone spent in a place, the higher the readings got. Maybe when the device doesn't get a lot of ventilation it starts to misread.
If you don't believe any of this naturally occurring phenomenon for some reason, there are several YouTube channels that check out these places, and try to find their own high activity areas/objects. There also subreddits dedicated to it,'and even a map where you can find places! ;)