For a good while now, my various hotspots have been stable, with only occasional "lockups" in RW state which I had typically attributed to rare events like power sags or interrupted updates of various sorts.
But around the first week in December, one by one my hotspots started stalling in RW states. Reboots restored them to RO status except 24 hours later they would be back in RW states. They would not flip all a once, but rather one here, two the next day, and so on. The change from RO to RW seemed to be happened at night (during the nightly updates?) but nothing in the logs seemed to be amiss or otherwise indicate exactly when the statuses flipped. So I created a logging script and dropped it into the cron.hourly folder (running at 17 after each hour) to see if I could narrow down the time frame.
But, while puzzling over this turn of events, I happened to notice that certain system updates (apt update/upgrade sequences) would also lock the systems in RW state. Annoying but not entirely unexpected. And then - coincidentally - a thread about "unattended updates" appeared in the forums.
Checking the "UU" directory strongly suggested that this is/was the source of the RW state: the UU updates continued for another day or two and then stopped - and so did the mysterious locked RW states. (Obviously, my experience here is based on the particular way I run/maintain my hotspots, so YMMV.)
Your general observations fit with mine: given what I see in typical system updates - the creation and deletion of numerous backup and temp files - it is not surprising that there are/may be some loose (open) files left around at the end of such machinations, especially if they are done "unattended".
More research is needed, obviously.
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My logging routine remains in place, as will my review of the UU files:
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logger -t "[$$]" "Pi-Star --> Disk Status: $(grep 'dev/root' /proc/mounts | sed -n 's%.*\(r[ow]\),.*%\1%p' | tr 'a-z' 'A-Z' | sed 's%RW%RW*%g')"
And the unattended update task continues on, but nothing happening since 12/12-13:
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pi-star@pi-star-1(ro):~$ lsll /var/log/unattended-upgrades
total 12
-rw-r--r-- 1 root root 0 Dec 13 06:39 unattended-upgrades-dpkg.log
-rw-r--r-- 1 root root 8265 Dec 27 06:33 unattended-upgrades.log
-rw-r--r-- 1 root root 0 Dec 12 21:22 unattended-upgrades-shutdown.log
pi-star@pi-star-1(ro):~$