The step ladder idea is a good one. Something, anything, setting there letting people know that you have already paid for the site.
It is possible that the people that backed in to the site did not even know it was occupied until after they were set up. Then they went to pay / post like I do and found your tag. They either threw it away right away thinking it was a old tag. Or, decided that since they were set up that it was too hard to move after that.
If you leave the site unoccupied you run the risk of something like this happening.