What to do?

[Jonathan Hutchins]

I'm with the "be vewy vewy quiet" folks on this one.  Do NOT go to the
authorities.

You gained unauthorised acces to computing resources, with the possible
exception of the open print server.  Just because a username and password
are stupid doesn't mean the system is legally "open to the public".  A cheap
lock on a door may be trivial to open, but it's still a lock.

If the print server was actually open and didn't require a password, you
haven't violated any law that I know of, but you're still engaged in an
activity that has been equated with terrorism by some major public figures -
wardriving, warwalking, and warchalking have all been called criminal
terrorist activities.

If you notify the owners and offer to fix it, it will look like extortion.
They could come back after you years from now.

However, the idea of printing something on their network printer that says
"your sysadmin is an idiot, your network is wide open" sounds like a hoot.
If you can handle the guy getting fired for it.