This is a very (probably, the most) likely scenario.
Bob, Tom and Bill get together for Form Xavier Gyros Unlimited in 1996.
Figuring that 'this new internet thing' might have legs, they register XGU.com for a few bucks and forget about it. XGU goes busto in 2001 and the remaining assets are divided up amongst the shareholders. None of the interested parties know anything about "domain names" and if you asked them, they'd probably think that if they ever needed XGU.com again, they could just go pay $10 and reregister it.
Given the absolutely epic rise in premium .com domain prices in the last 10 years, it's actually very common for people to own highly valuable domains and be completely unaware of their open-market worth. My brother, who owns precisely one domain name, registered a ridiculously good e prefix dictionary word in 1998 and has kept it ever since. The only reason he has a clue of its worth is because of my interest in domains and keeping him in the loop (as well as begging him to let me dev it). Without me, he'd probably have sold it to the first person who offered him $500. (although he does deserve huge credit for seeing the value in an e prefix in 1998)
It's like people who are on vacation and decide to buy a painting from one of the local street artists. They hang it on their wall and there it sits for a few decades, without anyone thinking much about it. In the mean time, said artist becomes very famous and his works highly desirable... last year, they were redecorating and decided they were sick of looking at it, so they sold it at a garage sale for $5.
It's a pretty standard tale of any object that sees dramatic price rises within the space of a generation. Often times, the owners who bought it when it was seemingly unimportant are completely oblivious to it's contemporary market relevance, due to the simple fact that it had no such relevance when they acquired it.
Another possibility is that the registrant passed away and didn't bother to tell his family about the domains he owned.