This was a common issue when I consulted with bankruptcy lawyers and had to figure out what domain assets the company had. Commonly the representatives only knew about some of the domains and we found at least a few more.

Same with third party services, sometimes they used one for something for a while and collected customer or user data there and then stopped but kept paying for it, and forgot they had it. We typically found these through analysis of their accounting.

Having a service crap out because someone didn’t pay for the domain is almost a trope. It never occurred to me that the reverse might happen - paying for unused domains.