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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.
Because they have no actual proof of it, and direct evidence against it. They talk about phones communicating with each other to restart... yet a phone inside a faraday box restarted.
It's almost certainly just a software bug in iOS which is why it's inconsistent.
It's also the most unlikely explanation of all, that phones brought in communicated with the ones in custody to reboot them. Even if it was a security feature to reboot phones under certain conditions, which is very likely TBH graphene OS does this, the best implementation isn't going to be relying on other phones randomly passing near by, it's going to be self-managed by the phone that reboots.
A remote command from some random phone to reboot does sound like the a wonderful vector for malware, though