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iPhones stored for forensic analysis unexpectedly reboot, causing problems for officials
(appleinsider.com)
This is a most excellent place for technology news and articles.
It probably requires root, but I'm sure Tasker can do this.
Really, all phones should have the option to reboot into locked mode on certain conditions like being taken out of network or being unattended for a while.
The interesting thing is if the manufacturer is shipping them that way by default.
Could possibly also look at the motion sensors to see if it is in a pocket moving around or if it was sitting around for a while.