I did, at one point, and it's been difficult to get out of that mindset.
We started talking when she was living with her last ex, and that relationship had been dead for some time (he was a drunk, they were basically roommates). After we finally got together, I found out that she had been hanging out with another one of her exes. She plugged her iPhone into my MacBook and her phone pictures popped up, and I saw some of her hanging out with him. Him in her room, him on her bed under covers. I know they were taken while we were together because a painting that had fallen off of her wall was in the picture. She later told me that he drove her home one night and followed her into her apartment before she kicked him out for getting too physical. She swore up and down that they didn't hook up or anything, but I never really believed that.
Another time, I found out that she did have a short fling with a guy she told me she had no history with, and that she sent him pictures of herself. They never did anything physical together because it was distance, but I know he was planning on traveling to see her. She told me that she wasn't interested in him, but he's always all over her pictures on Facebook and tumblr (comments like "I'm in lust with you" even though he knows she's in a relationship). It bothered me, and I told her that it bothered me that she wasn't setting boundaries for this relationship. She's a beautiful girl, so I know that guys are going to hit on her, but when it's so public it'd be nice if she could say something. She finally sent him a message saying "my boyfriend doesn't like...", and she seemed mad that the whole thing bothered me. "He's just a friend!" She says, but he's a friend that has seen her naked and, to me, it seemed like she was valuing his "friendship" over respect for our relationship.
And of course I've had my suspicions. I saw a recent text to a guy that wrote a song for her years ago that said "You take the best selfies" with a bunch of smiley faces. He always calls her darling, too.
So while I don't have concrete evidence that there's another guy, it wouldn't surprise me.
Which sucks to admit, you know?