With some digging on the gnome gitlab i've actually found what looks
like an existing bug report:
https://gitlab.gnome.org/GNOME/gnome-shell/issues/602
Looks like a fix was merged https://gitlab.gnome.org/GNOME/gnome-
shell/merge_requests/243 a week ago.
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Reinstalled the same NUC with 18.10, installed package updates &
rebooted.
Relevant syslog attached, detail starts at timestamp Oct 29 15:32:28
when installation was locked.
The stack traces appear to occur every 10 seconds once locked when
checking the output of "grep osdWindow.js syslog".
I can also confirm that this is lock-screen related as it starts logging
as soon as the screen is locked and stops when un-locked.
No notifications are evident.
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Desktop Bugs, which is subscribed to gnome-shell in Ubuntu.
I can confirm that this bug still exists in 18.04.
In my case in a completely fresh install with nothing added / configured
except for chromium being installed (and packages updated).
Installed Friday afternoon and came back in to work Monday to find the
disk full as per the original report.