From my experience it's either of two things. It could be a social jig that didn't delete itself when no longer needed, or it could be a piece of food.
The social jig sometimes shows up when you go into BuyDebug, but not always.
The food will be buried under the floor. It's slightly below the surface and that's why it can't be seen but it reserves that tile.
If you think it's a jig and you've got NRAAS Master Controller then click on the ground and choose NRaas | Master Controller | Object Stats | Not Inventory
Then click on the jig and confirm flush. The jig will have jig in its name.
I have made a video to show one method of getting rid of the food problem. If this is the problem, you will see a small fuzzy round shadow in the centre of the tile.
https://youtu.be/0gsTdw-NF6U
