I've heard of people buying hard drives specifically to snoop in them... but never an entire computer.
Though, you can never know when someone will be bored and try it out anyway... or maybe be trying to recover a file they deleted themselves, and end up finding old stuff of yours.
I would personally run DBAN
on it, just to be safe. Even the fastest, 1-pass method would make your machine virtually data-recovery-proof.
Odds are there is nothing to worry about... but I like to be on the safe side.