Hi @brice, @lv82 (and anyone who may bump into this)
After checking @lv82’s affected device, we found out that the underlying cause was an application script that failed to mount an SD card. The application code continued writing data anyway, but that data ended up in the container’s filesystem, (instead of the SD, because of the failed mount), which caused
/mnt/data/docker/aufs/diff to fill up.
So it looks like similar application-specific issue patterns are definitely worth looking out for, in cases where device storage is filling up unexpectedly.
Thank you @brice and @lv82 for helping us with the investigation.