OK SG my thoughts are that it is possible there's an OM. The last time my W had an A it was with a co-worker. I became suspicious when she started going to happy hours with coworkers very frequently and started coming home late. She rarely answered her phone. Like your W I had a bomb dropped on me before I discovered the A. According to her we were seperated even though we were in the same house.
I had the gut feeling she was cheating so I went thru her pocketbook one morning before work. I found a guys pager number and address. I confronted her and she denied everything saying he worked in the IT dept and she need his info for an emergency. Well I didn't buy it so when she was out I went online and checked her cellphone record and I was able to see where show was at because the bill used to show which tower calls were either coming from or going to. I had her then there was no more.denying the A. Plus she took our youngest S down to her work on an off day to have lunch w this creep. She even went so far as to have him call me and say they were friends that he would never do that and that he had a gf.
Once she owned up to it I told her parents and they were pissed. I even told her boss because her work frowns upon company romances. Two.weeks after telling her boss he was fired. She told me some.lame reason for his firing but I really think her work looked into things and prolly caught them.emailing or whatever. I even went so far as to spy on her after work and saw them together. So I did whatever it took to kill the A. I don't think letting it run its course works and it shows your weakness and lack of self respect.
I hope she's not doing it to you but it sounds similar to my switch a few years ago.
M 44 W 43 S 23 S 15 INILWY 9/11 Divorce Mediation started 3/13 June 30 the day W is moving out