/dev/loopX can show up in the indicator in some cases, for instance if the user installed some snap packages. Here's the "df -h" output on my laptop:
$ df -h
Filesystem Size Used Avail Use% Mounted on
udev 7.8G 0 7.8G 0% /dev
tmpfs 1.6G 11M 1.6G 1% /run
/dev/sda1 103G 70G 28G 72% /
tmpfs 7.8G 214M 7.6G 3% /dev/shm
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 7.8G 0 7.8G 0% /sys/fs/cgroup
/dev/loop0 7.5M 7.5M 0 100% /snap/hangups/3
tmpfs 7.8G 32K 7.8G 1% /var/tmp
tmpfs 7.8G 1.4M 7.8G 1% /tmp
/dev/loop2 6.4M 6.4M 0 100% /snap/hangups/1
/dev/loop1 121M 121M 0 100% /snap/ubuntu-clock-app/5
/dev/sdb6 184G 146G 29G 84% /partition2
/dev/sdb5 184G 160G 15G 92% /partition1
/dev/sdb7 177G 126G 43G 75% /partition3
tmpfs 1.6G 160K 1.6G 1% /run/user/1000
Furthermore, since the /dev/loopX devices show up as full, this triggers the "Check your disks!" message in the indicator.
Thanks for mentioning! I am not at home at the moment, but I will fix this after the weekend. Looking at your bug report, it should be easy to fix, thanks for that!