A file in a directory has a Group ID "501"
After checking /etc/group, Group ID "501" was not found
how is that possible ?
Perhaps a group with that ID existed at some point but was since deleted?
It's also entirely possible to, as root,
Did the file originate on a different machine? You often see this happening, for example, with unpacked tar archives that were created elsewhere. The user/group pair existed on the remote site, but possibly aren't defined locally. Therefore, utilities that try to resolve user/group numbers to names fail, and fall back to displaying the numerical ID instead.