Yeah several of mine that travel around have gone missing, but others that seem to move pretty regularly. Interestingly I've had a couple that were missing for a year or more show up again in the last couple of months, go figure! But people logging them said they found them in a different cache than what they showed in. I think maybe sometimes that people just forget to log a coin out of a cache and move it, or they don't realize they are supposed to do that if they pick it up. The rest probably horde them!