I Think I Stole Something from the Public Library

So I was at work and was going to listen to a CD - which is not something I do that often - so I opened the disk drive on my PC. There was a disk already in there - one I'd gotten from the library. And returned to the library. Or, so I thought. Um, oops. I had absolutely no recollection of even listening to that CD at work so I have no idea how long it was in there. It's been at least a couple of weeks since I returned it. Or, returned the case anyway. I'm not sure what to do next. My account says nothing about me still needing to return it. As far as the library is concerned, I returned it. I've checked items out since then and haven't been asked about it. Did they not notice the empty case? Or, did I put something else (one of mine) in the case? Has it been circulating with the incorrect CD in it? Wouldn't someone notice? If I did return it now, would I owe a ton of late fees once they figured out how long I'd had it out? It raises all sorts of questions.