I have no idea about whether or not your wife is doing anything inappropriate, but i will say that i have a lot of matching bra and panty sets that i don't really consider lingerie and i do wear them for myself, not just for my boyfriend. I also have true lingerie that he's never seen because I've bought it but just didn't end up feeling confident enough to wear it or am just saving it. I also have some borderline sexy matching PJ sets that are lingerie-adjacent, and I'll wear them on a business trip just because it feels like fun and fancy and adult to lounge in a hotel room alone in something sexy. I've never cheated but your exact situation could have feasibly happened to me.

Not saying there's nothing nefarious going on, but her explanation about the underwear would track for me. Sounds like there's more that's setting off alarms for you though.