/etc/debian_version states unstable and the machine has been picking up updates then it will be tracking recent updates not something as old as potato. Unstable is a "continuous release" arrangement, more-or-less. Check the version numbers of some known packages, that will give you a better idea. Don't just trust one program/libraries version number though as some may have been installed "by hand" so will not be tracking the standard repositories. If the admins have not been clearing out
apt's cache for space reasons that would be a good place to check - it usually live in
ls /var/cache/apt/archives/ -lhrt should give you a list of packages currently cached ordered by the date they were drawn down from the repository. if you don't find the cache there (rather than finding it there but empty) search the machine for
.deb files - the previous server admins may have moved the cache elsewhere.
If you don find the machine is running things as old as packages found in potato then I recommend against trying to upgrade it. Backup all data and configuration and start from a new install.
If it is running unstable I would suggest a reinstall also. Unstable is surprisingly stable in my experience but breakage does sometimes happen and things can sometimes stay broken for a couple of days (in these cases workarounds are usually available in a shorter amount of time, but it is still risky) - so it is not recommended for production use.