Yes, they do go missing! About half of our travelling trackables - that is, excluding our vehicle and our name-tags - have gone missing. And we agree, the reasons include inexperience, lackadaisicalness (yes, it's a word!) and maliciousness! With respect to one of these missing trackables, we sent e-mails to all the cachers who had visited the last cache where it had been placed to ask about it and a number of them answered but some didn't - which, of course, narrowed down the list of possible delinquents - but that was as far as we got in tracking it down.
On a brighter note, we had a trackable travelling in Europe - in Germany, in fact - and it was in a cache which was destroyed by a violent storm in which trees were ripped out of the ground and so on. We know that, because the owner of the cache wrote us to tell us the fate of the cache - and, of course, our trackable! He spent some time searching the area for the cache but in vain. In due course, we arranged with him that if we supplied a replacement, he would place it in a comparable cache in the same general area and it has travelled on since being placed. So that was a very positive experience - and we now know a cacher in that part of Germany whom we would have no hesitation in contacting if we were going to be in the area!
The trackable concept is a great one and it is fun to follow them. So we continue to put them out from time to time but we spend less time than we should, probably, in keeping track of them!