Thank you for the explanation

* does it happen overnight but not in the day?

No, not necessarily. But it only happens when my phone is locked - never when the screen is on and as far as I can see it doesn't depend on the charger - it happens both while charging and while not connected via USB

* does it happen if left on with no auto-TKlock?
I think I should try this.
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I used to have Shortcutd installed for some time could be related? I guess I'm out of ideas. Right now I'm going to unistall everything except FBreader and see if it happens again.