Currently sat here reeling. Shaking so much. I just didn't expect it. Perhaps I am stupid. 
Last Oct DH left his phone unlocked. I casually picked it up and had a scroll through. No idea why I did this as not suspicious or anything. Found a set of messages back and forth - about 5 or 6 in total. Basically saying he was thinking of her today and did she fancy a drink soon. That sort of thing. He definitely seemed keener than her.
Then this week found a series of messages after a works night out, with a different woman. He had been out and it appears he met her whilst out - it is unclear whether they knew each other before. Basically they are both pissed. He says he hopes he wasn't inappropriate and she says no not at all...
Anyway this evening I have been digging about in his online history and found a whole set of messages from 2013 and 2014. Between him and the first woman (Oct). It appears they have kissed, although I don't think anything else happened. This is because she stopped it. He was clearly very keen - talking about only wanting her and sending her chasing messages about meeting up and so on.
OK wise ones. What do I do? He is currently away with work.