I'd ask her tonight if she had fun... maybe see if she wants to talk about her evening out... LISTEN.
And then I'd casually say... "the next time you go and you are going to be late, please call and let me know, so I don't worry." Don't buy the "I didn't want to wake you or the kids," reply. I am SURE that you fully expect your D18 to call if she is going to be late, regardless of what time it is or who might be asleep.
Other than that... my antannae would be seriously up, if I were you. Seriously up.
