So after a rush of customers this afternoon, which means I can't pin down the culprit, I went into the bathroom and found a bra lying on a chair. The restaurant has just one bathroom, one stall for both sexes.
It was a fairly new-looking bra -- expensive too, by Alfred Sung. It was white with embroidered pink flowers and green vines. Probably a B cup.
I told my boss (he's gay and is very cool about these things) and he dashed off to the bathroom. He came back out and said "I thought you meant it was in the toilet!" I went back in and picked it up in a plastic bag, and when I came out of the bathroom L took the bag and hung it on the back of the door to the basement. "Maybe someone will come back for it."