Your daughter tried it on and put it on the counter. Then your son put a towel over it. Your housekeeper put the towel in the hamper taking the necklace with it. She then remembered that you have instructed her to let your son pick up after himself so she placed the towel back on the counter.
Unfortunately for you this led the necklace into the hamper and from there into your washing machine. You will find it there but it may have been damaged...