I noticed the water level being lower than usual so i topped off the pool. Then when i turned the hose off, i noticed the level come back down again within a day or two. The level is now about 4 inches below the skimmer openings so i think i can eliminate the skimmer as the source of the leak.

Its leakingnwith or without the filter running so i dont think its the plumbing lines from the skimmer to the filter.

The only other thing it could be is the main drain, right? How can i test this? What should be my next step?