P
Well, there are a LOT of grids. There are reference, parent and artboard canvases. They all can have grids (they have a little grid icon next to them in the Canvas & Artboard panel). Furthermore, all these canvases can have any number of artboards and they all can have grids. Phew... So even if you delete all the grids (in the Grid Editor) for all canvases and artboards except the artboard you are working on, you'll still have this issue. So not sure where the extra set of grids are hiding, if there are any.