I think you are on the right track. The leak should be where the water level stops.
You can rule out the return lines if the rate of water loss does not change with the pump on or off. And you can rule out the suction lines if you do not get a lot of air in the pump basket when the pump is running.

Skimmers are one area where leaks can develop so even though you haven't found anything yet, I would focus on that area. You may need to raise the water level some and then use the dye so you can actually see it exiting a gap somewhere.