Well I never restart my iPhone, from time to time it requires me to put in my pin code, as though the device has restarted at some point during a long period of an activity. Note that this happens at other times, other than overnight, I tend to use sleep monitoring software that runs tasks overnight so the phone never restarts at that point, it finds another point during the day to do. It’s internal updates. I’ve rarely if ever seeing the iPhone crash restart, I have pushed the limits of it before and sometimes it nearly locks up, but that is an unruly app normally, this is an older SE 2020 model.
iOS requires you to enter your passcode roughly weekly, even when Touch/Face ID is enabled. This is not due to a reboot, it's just an added security layer. I don't believe that the phone is in a BFU state when this happens.
HOWEVER... it appears that Apple added a security feature in 18.1 that reboots the phone if it hasn't been unlocked in 4 days. It makes me wonder if the reboots in 18.0 were this new feature malfunctioning. In any case, this behavior (and the bug in 18.0) are probably what police are seeing.
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Came here to say exactly this.