I put the Pixel in a bag. Possibly the lock screen was not activated. Possibly there were touch events, possibly apps were started or entries were made in an app.
When I picked up the Pixel again hours later and wanted to use it, the PIN entry screen appeared as normal. I entered the correct PIN as normal, but then nothing happened. This is reproducible.
Finally, I realised that I had to press the button at the bottom right of the keypad to continue. This was never the case before. When I did that. The message appeared: "too many login attempts, wait 11 seconds".
I have since tried restarting, switching the phone off and on again, entering a random PIN with more than 6 digits. Now it says "Too many attempts, wait 900 seconds".
When I switch off the device, the menu does not show 4 options as usual, but only 3: switch off, restart and emergency call.
The device has activated developer mode and was recently connected to Android Studio. USB protection is deactivated or at a low level, restart after inactivity is not activated. There is a duress password and keyboard scrambling is active. The current Graphene stable-channel version is installed.
My thoughts are to try to enter the PIN via a USB keyboard, or to enter more than the 6-digit PIN, i.e. to enter either random digits before or after the PIN. Or to install an app via adb or Fastboot that enters the PIN in an alternative way (is there such a thing?) or to zero-out a partition or to wait until the battery is empty and the device has forgotten status data and restarts cold.
A factory reset is out of the question, I have captured important data in the last few hours before the incident. I would rather put the device away until a zero-day is found that I can use to hack into it myself.