Probably unwise. "What else did he/she do?" types of questions come to mind. Probably depends heavily on where you're working, though.

A little paranoia can be a good thing.

At a previous company, we had a culture of sending out prank emails from open machines.

I once sent out an email from a co-founders account which said that he was fed up with the crappy codebase and was hiring a new team to rewrite it from scratch and that the other (non-tech) co-founder was to take over the existing tech team. No one took it seriously (non-tech co-founder helped me draft the email, the other one laughed while reading it after the fact), but there was a board member in the mailing group who thought it was serious and started sending panicky emails to the co-founders.

That was how we generally accomplished that enforcement -- see an unlocked machine, write an email to the team list saying "I'm bringing breakfast tomorrow!"