shrimp need iodine in their tank to recreat their shells after molting. i could jsut be that you didnt notice it molting. They will go deep into liverock or corals somewhere in hiding (because your pretty vunerable with no shell) and will stay there for only a day or two to molt. So the old shells jsut could have been burried somewhere in your live rock or corals if you have any and you just might have not noticed