I would agree with this it's her time and her fun, if she wanted you to know she'd have told you.
The first time my wife stayed out all night she was supposedly going out with the girls, she phoned me at 11pm and told me she wouldn't be home, she was staying with one of the girls, stone cold sober,I told her she better get home and she just calmly said 'No, I'll see you tomorrow'
We had a blow out the next day but the argument caused so much upset I decided from that point on just to let her do her own thing.
When she went out with her slutty married friends she always came home legless,this night she was sober, I believe she was seeing a work colleague and that's who she was with that night
Ever since it's been very much a case of......