Guys, I found strange panties in my boyfriend's room?

My boyfriend and I have been together for a few years now. He was on a guy's holiday last week so I decided to redecorate his bedroom for him with the permission of his parents (we both live with our parents). When I moved his bed away from his wall something fell down the side and when I pulled it out it was a pair of black pants. I won't go into the gross details but they had definitely been worn, I'm not sure how long ago.

Of course I confronted him when he got back, he told me they must have been his ex's but we have been together now for years, I am sure he would have found those pants by now?
Guys, I found strange panties in my boyfriend's room?
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