I caught my male au pair sneaking a girl out...

I'll be honest, it was kind of fun to introduce myself and make a point of saying they should use the front door. (I'm going to be such an obnoxious mom to teenagers!)
I don't have a problem with visitors/overnight guests/or whatever unless I'm suddenly expected to feed three additional people that I didn't plan for, but even then I would probably just grumble to myself rather than say anything unless it became a pattern. I do know that we are often the landing pad for au pairs that live in the far suburbs, and I would rather they do that than make questionable decisions late at night.
The longer I'm a host mom, the less I care about what the au pairs do and where or when. My kids honestly don't pay that much attention to who comes and goes and when, and the kids have always done co-ed sleepovers themselves so they don't think anything of it...