That exact thing happened to me. I found all of the caches in the park, so I went hiking around looking for places to hide one. I spotted a hollow tree, which was a perfect hiding spot for a cache, and went up to it, and saw that there was an ammo box in it. The cache wasnt found for a long time. I went home and found that it was the final part of a multi, and the first part went missing, and they didnt get around to replacing it.