The company believed that some external media was not there anymore, and found it months later exactly in the place where it belonged.
If I was bribed to give a copy of that data to someone, I'd take the media, copy it, sell it to the briber, and return the media where it belonged. Except in this case, everyone is up in arms because the media is gone missing, so I wait until everything goes quiet, and that's when I return the media, a few months later when nobody is watching anymore.
Conclusion: If the external media cannot be found, and then some months later it is found exactly where it belongs, then it is quite likely that it has been copied in between, so that should be treated as a breach.
On the other hand, if the media had been accidentally destroyed (like putting a CD into a broken CD reader can destroy it), that would not be a breach. You'd have to check the exact wording of the GDPR to see if such destruction needs to be reported, but it wouldn't really make sense.