This has been going on since I started the game; whenever I run over that exact spot of land, it sometimes collects random crops. No matter that I have the coop there currently, it happened when I had tilled land there as well. However, there is nothing on the ground ever.
This mostly happens if/when the auto-harvester slots are full, and the crops would fall on the ground beside it. But they are outside of the greenhouse!
Steam version currently: v1.1b-1235
Edit: Playing keyboard and mouse