In the 10+ years my mother has been in a nursing home, I've run into this also. Sometimes the labels come off clothing, so there's a huge stack of clothing in the laundry room which I rummage through occasionally. I must confess that I rummage through it less and less now - I've reached the point where it's so much easier for me to buy new clothing.
Sometimes clothing ends up in the wrong room, through simple human error. I have found other people's clothing in my mum's dresser and closet and returned them. I suspect that items of my mother's have ended up in other closets and dresser and when that person died, the family cleaned everything out without really looking at things.
I've long since stopped troubling myself about things going missing, as I have removed anything of real or sentimental value. It's only clothing and I enjoy buying Mum new clothes. I triage through her clothing regularly and get rid of clothes getting tired and worn and buy her new things. I want her to look as nice as possible.