Found a used condom? Help

So my boyfriend and I have not had sex at his place for a little over a month. However, tonight, I smelled something fishy, opened his trash can, and found a used condom with paper towel wrapped around it. Covered with yellow sticky, old semen. He says he smelled it a few weeks ago and he wasn't sure what it was. I didn't smell it last time I visited. So did he really leave that mess in there for a month, or is he cheating? (Would it have dried out after a month?)