My iPhone 6 is running iOS 9.1 (13B143), not jailbroken. Installed one app, f.lux, which is sideloaded, everything else is from the App Store.
Occasionally, about once per day, while charging or after unlocking the screen, the phone will present the "Enter your passcode" screen.
Just to be clear: I have never set a passcode, yes I know the "skip" step during upgrades, I do not have roommates or other people touching my phone, I do not use Touch ID or Apple Pay, and I have never set a passcode.
The passcode screen includes a full QWERTY keyboard, not the digits one. And entering anything in there will just shake the screen (incorrect passcode). I have not attempted experimenting with 4 or more incorrect attempts in a row.
The workaround is to simply lock and unlock the screen once or twice and then slide to unlock. This method will unlock the phone without a password 100% of the time.
Has anyone seen this before and can it be cured?