If you dig deep enough, eventually you'll find something. Everyone will.
Yes, I have snooped on my partner before. It ended up with me ending the relationship due to me finding out about him cheating on me while I was out of town. It's one thing if you snoop just for the sake of snooping. However, in my case, there were plenty of other warning signs that left enough of a question in my mind as to what he was up to when I wasn't around that it led to the snooping and as I stated before, me breaking up with him.
In my case, it was a cell phone bill that was about three times the size that it should've been. After looking deeper into the bill, I discovered that it was all multimedia messaging (picture mail) on his cell phone (shared bill). After looking at the pictures, I discovered a bunch of nude photos that he sent to somebody and after contacting the phone number that they were sent to, I found out that he'd been hooking up with this guy behind my back while I was out of town.
It is never okay to invade somebody's privacy, no matter what the reason. However, in my case, there were enough other red flags that my trust was lost and I feel I had the right to investigate the bill, considering that he was not sending pictures of any kind to me. To this day, I feel guilty for investigating as far as I did, but, I also feel like it staved off a possibly tumultuous life with this scumbag.