I didn't see any of it at the time either... because I trusted him implicitly. he always had his phone near him for work, and if a text came through I presumed it was to do with work, because I was too trusting. He had the phone on silent all the time too.
After he left for the second (and last) time, I checked his facebook, saw the flirty chat between them - then checked his mobile bills online. I had access to his account because I did his accounts for him. The same number appeared over and over and over, and I checked it and realised it was OW. i was shaking like a leaf. Suddenly, everything that had seemed so confusing, all became a bit clearer... although I didnt want to believe it and defended him on here to the hilt. I said - but she is married to his best mate, he wouldnt do that to him, because I know XH so well, I just know that nothing is going on, he is supporting her blah blah blah...
I was just totally blind to the fact that something could happen, because I trusted him so much! But the very fact that he hid it all from me, proved the point, because if it was a genuine friendship, he would have told me about it, not deleted emails, texts etc.
I remember being on holiday and in the car he put the phone in the side pocket instead of the middle of the car as usual. He charged his phone beside the bed, not on the kitchen worktop. When he left it in the bathroom once and I went in there, he got really arsy with me, because I was, err, in the middle of something, and couldnt pass his phone straight out to him. I assumed it was because of work, but of course he was afraid that a message would come through for her.
He sent her motivational emails, things like Baz Lurhman's Sunscreen to listen to, kept telling her that everything would work out ok in the end........
all this, while leading me on pretending that he was trying to work things out within our marriage. He had me walking on eggshells, afraid to upset him in case he walked out again, and ALL THE TIME, he was in contact with her...
I only ever managed to see one text from her and it was telling him how sweet he was.....
she also seemed like a nice girl and was supporting me through our breakup

sorry to go on... 
I think that of course men can have female friends and vice versa, but you must be totally honest, open and above board about all and every bit of contact.