How do IM contacts show while chatting?

[Lori]

I found a secret gmail acct my fiancee has and to my horror, it is chocked full of emails and IM's to women he has met in onlline social sites or from craigslist, etc. He even posted a pic of his hard thingy on there. Anyway, my main question is about his IM contacts...while I was on the email as invisible, I could see his name and another woman's name keep popping up in the lower right hand corner...like he was IM'ing her at that very moment. When I confronted him about all this and asked him aboput the woman, he admitted it but said he hasn't talked to her for months. I told him what I saw with the names popping up and he denied he was online with her at the time and says he doesn't know whey the two of their names only kept appearing. So, if he was logged on his email at his job and chatting with her, and I was logged on but invisible, would I see that kind of stuff...who he is chatting with?