@Ilana85 Sometimes towels just go missing, normally a guest has packed them without realising, not stolen them ! Sometimes, the guest gets in touch when they get home and offer to post it back, but I am afraid I find this is just part of hosting. I have never asked a guest to return a towel and I find it is just best to accept that once in a while a towel will disappear.
I am afraid as you don't actually know which guest has packed the towels there is very little you can do about it.