I am a regular poster but have name changed for this just in case...
Basically I found a random condom in DH's drawer, wrapped up and hidden. I asked him about it and he said he bought a box for us to use a few years ago (when I was having contraception issues) and never told me. There were a few missing as he uses them for 'posh wanks' hmmm
I then asked him if he had anything to tell me etc, I was putting pressure on him and he admitted there was a time a few years ago when he was really attracted to some other people (in a night out situation) but they knew he was married so nothing happened (basically because they wouldn't) 
He did admit that he had a two day flirtation with a girl and it ended up with heavy petting & BJ outside on a night out but nothing more.
He said he sometimes carried a condom, just in case, apparently he wasn't looking for anything to happen but better to be safe if it ever did!
What the heck do I do with this! We did go through a bad patch but things have been great for the last few years and half of me just wants to forget he told me and move on, can I really do that. He didn't do a great job convincing me nothing would ever happen again.