The fountain may be adding to a lot of evapouration. The big thing, though, is you have a LOT of plants and the weather lately has been sunny and hot. Aside from the water literally just evapourating, you're getting a lot of transpiration to keep those plants cool (i.e., the plants are using much more water than normal). I have pots outside I water in the morning until they're completely soaked, and when I come home from work, the plants are drooping, the soil is dry and cracked ... They used that much water.
That's my guess, without looking at it. Liner leaks happen, but they're not as common as you would think. It's certainly possible that a rock got pushed down, that a sharp-clawed raccoon was hunting frogs, etc., but -- IME, it just doesn't happen all too often.
Assuming it isn't just evapouration, I would stop filling the pond and let it continue to drop. That'll pinpoint the level of any possible leaks. Hopefully it's along the side and not in the bottom.
In the meanwhile, check all your plumbing and such, too -- 90% of real leaks in a pond occur in a cracked piece of pipe or tubing (usually located under an incredibly heavy rock that you don't want to move, waah!
). I've had a number of pond clients with ponds that were apparently leaking at the very bottom of the liner, only to discover that it was the line running to the filter that was leaking. The check for this is to turn the filter off for a day or two and see if that changes the leak rate. (Obviously, use caution if there are a lot of fish in there).