I was out of the game for quite a while now, 2 years probably, i've logged in yesterday to notice that all my characters are gone, both on Icecrown and x1 realm. I've checked character restoration tool and couldn't find any of my characters on the list.
If they were removed in an inactivity clean up, due to being under the minimum level and your account not being Premium, they're gone for good.
Restoration is only available for 30 days, though, so if someone manually deleted the characters longer than that they also wouldn't show there.
Level 80's wouldn't be removed in an inactivity clean-up and that's the only sort that has been enforced by us on Icecrown and Lordaeron, so yes, someone must have accessed the account to delete them manually.