You have checked everything. There are no liner holes, the waterfall water is falling in the pond and not sneaking off the back, all hoses and hose connections are tight and secure, but your pond is still losing water.
Do you have a spitter? A spitter is some ornament, often a statue of a frog or bird that has a hole in the bottom and spits water out of its mouth. I am not a big fan of spitters, but you are and you have one. First disconnect it. If the pond water level stays put, you know that your spitter is the cause. Often as pumps get clogged up or hoses get full of algae, the water flow slows down and the spitter drools. When that happens the water sneaks out of the pond, sometimes only a drop or two at a time. If you don't think that is very much, let water drip slowly out of your inside faucet and see how much your water bill goes up.
To repair a spitter drool, you can move the spitter so it sits totally inside the pond or prop it up next to the pond with a serious tilt into the pond so water can't escape. Watch out for this method because the thing can fall into your pond and puncture the liner. Then you have bigger problems. My solution. Disconnect the spitter and let it sit wherever you wish minus the water coming out.