While testing the recent micro-evtd changes I noticed that this problem
still happens in buster and bullseye. After some hours or days of
uptime, the box sometimes beeps and runs the fan, for no apparent
reason. Sometimes it beeps once, other times continuously.

I think these are the same root cause:

https://lists.debian.org/debian-arm/2017/02/msg00038.html
https://lists.debian.org/debian-arm/2019/04/msg00005.html

The symptoms might vary by hardware.

I set DEBUG=2 in micro-evtd.conf and confirmed that micro-evtd is
causing the beeping, e.g. /var/log/micro-evtd:

Wed May 19 19:21:46 PDT 2021 B 200 0 micon
Wed May 19 19:21:50 PDT 2021 B 201 0 micon
Wed May 19 19:21:52 PDT 2021 B 200 0 micon
Wed May 19 19:21:54 PDT 2021 B 201 0 micon

I am not running with iomem=relaxed, so micro-evtd uses the fallback
button detection method instead of /dev/mem. I think there is something
wrong with the button detection in this path.

I can't remember if we ever discussed whether there is some modern way
of accessing the GPIO? would it require more kernel changes?