I was going through DH's mobile messages, as you do and came across one to a female barmaid friend who works at a pub that DH frequents... frequently
one of the messages was a link to a song and a message to the effect that he considered buying two tickets for them to attend this concert, but that he thought they probably both wouldn't have time.
There are other (text) examples of an emotional relationship although said barmaid currently has a long-term live-in boyfriend.
I haven't confronted him on this although I have previously over previous texts to the same woman. (lots of kissy emoji and long hugs and mentions of lovely taxi rides home from pub and him saying he only gets to relax when they have time to chat.) By contrast we barely say a word to each other at home, he plays pc games all night, and when we go out also fairly stilted conversation between us.
I should mention I once bumped into them having a morning coffee at the train station, but I didn't make a big deal about it and DH mentioned later that she had had issues other WAG's of pub-goers over being 'open and friendly'
I'm just not sure what to do next, we have 3 DC and earn similar amounts at our jobs but I think it WBU for me to just up and leave over this, wouldn't it?