<human>: Can you condense the text into a few key points?

I've been with my BF for about 10 months, we recently (3 weeks ago) moved in together. Everything is generally awesome (from my perspective), we're very much in love, have great sex nearly every day, love spending time together, etc. etc. 

This afternoon I was going through some of his clothes looking for a matching sock when I found a pair of panties that are DEFINITELY not mine (wrong size, never seen them before). 

I'm completely taken aback. I really honestly don't think he's cheating, I'm pretty sure they're his ex's because we moved into a new apartment together 3 weeks ago and it's what looks like a bag of old clothes that haven't seen the light of day in a while. At the same time I have major insecurity and trust issues, plus, even if they are his ex's... still not something I wanted to run into.

Possibilities wrt the origin of the panties:

1. He likes wearing them. (Possible, but we practice BDSM and are very sexually open with each other)

2. They are his ex's and he never got rid of them. (Most likely)

    a. for benign reasons (bag of old clothes, possible hasn't gone through them in months/since they split)
    b. for not so benign reasons ("memento" of her/their relationship, they didn't really break things off until much more recently)

3. He is cheating and they are the other woman's (Possible but very unlikely, we spend 95% of our free time together)

I'm at a loss of how to approach this because I'm not sure if it's something I should even be concerned about and I know for a fact whatever the truth is (unless he's cheating which I doubt) I am probably overreacting. How do I approach him about this in a non-crazy OAG way? Or should I just bury them in the clothes bag and pretend I never saw it?
<bot>: Found an unknown pair of panties mixed in with my bf's clothing, should I talk to him about it or just pretend I never saw it?