How to check if you have stealthy random reboots. Post results here.

My Z3 sometimes does "stealthy" auto reboots when not in use. The system crashes and does an auto reboot but not a full one because the phone won't ask for the sim pin code, it will reboot to the lockscreen so unless you're looking at the phone while it happens you won't notice. If you're lucky enough to catch the reboot, you'll notice the screen suddenly lighting up, and the boot animation will play, then the phone will go back to the lock screen where you left it. My phone does this a couple of times a week, most of the times I don't catch it but there's a way to tell: go to Settings → About Phone → Diagnostics → SYSTEM INCIDENTS

If it shows 0 this means you haven't had a reboot in the past 10 days. Right now on mine it shows 1 which happened during the night (so I didn't catch it happening). I always have 1 or 2 in there, I'm not sure if it always happened, but I started noticing it after Lollipop, and factory reset didn't help.

Please let me know if you have any "system incidents".