Ignore it. Comes to terms within yourself, realize and accept that this part of the journey's replay phase. Then ignore it.
If you make a big deal of it, it will probably only push them together more strongly. If you flip sh!t on him, you will justify everything in his mind most likely, and make the OW more attractive. Try to think of it as a drug (which its similarity to how drugs alter the body and brain chemistry is remarkable..). OP is a symptom of the disease...
I would get some boundaries lined up and ready if it is what you think, though...for example, I had to tell W that the computer and webcam was NOT to be put in our bedroom, and that I would appreciate her turning off her speakers so I didn't have to listen to the IM chat notification sounds (they still cause me to cringe and can bring up anger, sadness, humiliation feelings, even though I know its the kids on fb and not W on yah00).
Really dig into the posts here on this topic, even over in infidelity section...lots of wisdom and coping strategies. I would also check out this ebook that really helped me with W's 1st phase and old high school bf affair...google "breakfree from the A" ...helped me have understanding, and strategies for the different types of A's.
I learned the same way you did to not snoop, not good for US...what bothered me more was the emotional and "trying different personas" things, much less than the physical.
Hang in there! Keep posting, get it out here.
T^2
In the depths of winter, I finally learned that within me there lay an invincible summer. - Albert Camus
Uncertainty is the very condition which impels people to unfold their powers.-Eric Fromm
