Coming across someone's private keys does not mean they have no copy of it stored away. They could still have access to it, and you do not need to take any action to finding them, except to inform them that their assets could be compromised, cause if you could have come across it, there is a possibility that someone else could.
I suppose you could send a dollar's worth to a different address and hopefully they'd get the hint.
A hint that it was compromised?
- Jay -
Yes, I'm assuming that they'd still have their key in question.