At one point, I was scouting a place to hide a cache in one of my favorite, little known, local forest preserves. Found a decent spot, in a hollow tree - not too bushwacky. Went to put the cache in, and found another cache! Pulled it out, pristine log. Hmm - I had checked before I left, I knew there were no published caches nearby. I signed the log, put it back and hiked away the requisite distance to hide my own. I figured it was bad timing - and they would both show up on GC. Never hear from the other one again. Checked on it some months later when checking on my own, - and it was gone.
Still wondering...