How did you check the fluid in the brake reservoir? Open the cap, or shine a light through the body, etc? It's hard to tell easily (at least for me) from the outside because the plastic is so thick, and the MAX/MIN lines are not marked in a contrasting color on the Edge.
Anyway, it should only be "empty" if you have put a lot of mileage on the brakes and the pads are ready to be replaced, or there is a leak in the brake lines somewhere. You should also notice a decrease in braking performance. If it is indeed low, you can try filling up to the MAX line again, and watch it to see what happens.