Can the Groundspeak system handle a “retrieve” log only? This way the coin would show up in the cachers inventory but would never show up in a cache because no “place” log is entered once the coin is placed into a new cache?
This way the coin wouldn’t ever show up in a caches inventory for the coin thieves to target, they would have to get lucky and stumble across them.
The cacher placing the coin could, like trowel32 suggested send a courtesy email to the coin owner so he/she would know were the coin is.