Im not in to sly checking of texts or emails tbh.
Aside from that - his doesnt sit playing with his phone, its not hidden away, and i cant remember the last time a text message came through on his phone. He often gives me his phone to use (its a phone in a pda) which he wouldnt if he was hiding something.
Anyway, had a chat with him when he came home from work last night.
I asked him how his day was etc. I cant remember if i asked or he said about the knickers tbh. He said that he pulled them out his bag, waved them about and said "okay, so whose idea was this?" he said he made out like it was the first he'd set eyes on them. No-one owned up to it at all, and just made jokes about him playing away "aye aye" and all that sort of thing. He said he will wait till his other colleague returns to work from sick leave (he is quite good friends with this guy) to see if he is in on the joke.
I told him that he needs to find out because they might find it funny but i certainly dont. He said "i know, it could have potentially ruined our relationship. Im not happy about it either".
After that, i said to him that it occured to me that he might be upset about how i reacted. He said he was shocked rather than upset. (and made an apparently 'throw-away' comment about me not thinking he was attractive enough to be able to have an affair 'ha ha')
He was thrown by the fact that i just came out with the comment about finding some other womans knickers, and then carried on like nothing had happened and he didnt know if i was joking or what. He expected that if i was being serious i would go mental and for us to have a huge row. But i didnt. He would have understood if i had been really annoyed at him but was glad i wasnt.
I said i sensed he was upset all day about it. Again he said, more confused by it all, and a bit angry that someone thought it would be an amusing thing to do. He was concerned about how i was feeling about it - he thought id want to discuss it. I told him that i know him well enought that he goes quiet when he's upset, but is very vocal when angry.
I explained that i had no grounds for suspecting he was having an affair - i just found a pair of womens knickers. Albeit dirty ones. He said he wouldnt ever do that, and aside from that he said he doesnt have the time! I agree with him there.
He mentioned that when he did a day trip to one of his company's new sites last week (the company he works for has just bought out another company) he said he didnt sense any hostility from them, but doesnt rule them out as having done it as he doesnt know them and thus their humour.
I told him not to drop it with work, i think its an appalling joke and it could have had dire consequences. He said he wont - he will speak to his colleague when he's back in.
So, thats about it really. Anyway, thank you everyone for your support, advice and comments. They have all been helpful.