I know these things happen to a lot of people, but from what I heard about rumors and in fact seeing people post on forums themselves... lots of things you wouldn't believe happened in the past 3 years you were gone from RS.
First, over the past 3 years, there was a super recovering random accounts frenzy organized by one of the blackmarket forums which is now forced closed and controlled by Jagex, but Jagex couldn't really do anything about it before that. (They did change the recovery system and how it works a few times though throughout the past 3 years)
But from the information above, I'd say nearly 90% of most accounts that haven't been active recently was recovered by total strangers who took information from facebook, twitter, google, any free source of information they could find. And basically. It become "their" account.
Lots of partyhats came into game this way by recovering random accounts, (another reason why partyhats dropped in price in RS), and lots of rares come into games too.
The massive recovering frenzy also recovered many 2001 level 3 accounts which became (9HP accounts), and that's another reason why there are still so many 9 HP accounts available for sale even today. You can't say they just suddenly remember their username and password to an account created nearly 10 years ago. That's not going to happen, because they will NEVER remember stuff 10 years ago. No one does actually. Unless you happen to HAVE a slip of paper with your account details on it you WROTE 10 years ago. Which I don't think will happen either unless you live in a shabby shack. So with those two possibilities canceled. You could most nearly guess where the 9 HP accounts come from. From randomly recovering accounts. That's right. I'd say it's illegal, but Jagex has got to stop this.