I'm curious, have you left your waterfall turned off during the last day or so, while you are watching the water level? If not, shut it off and wait and watch. I assume you have nothing running and pumping so that isn't possibly the problem. I agree with Epidemic, run your hand over every square inch of that liner to feel for indentations, and push on it so that if there is a crack that is sometimes sealing back up, then getting pushed from the outside dirt or sand and opening up and leaking, you will have to push to make it open up to feel it. I think if you have some type of large tank, put your fish in that and pull that pond out and check it over well. Also, if you could pull it out, you will see where it is leaking. That's how I found my leak, where it was wet on the side of the the pond. In your case, it may be leaking straight down and now coming to the surface on the sides, though. But, if you pull the pond out of the ground, you will see exactly where it is leaking, or at least the general area, and have a better idea of where to look. Of course, if the whole area under the pond is wet, then it's a major search!
I think it's funny you have a lifetime guarantee! Bet there will be stipulations, although I hope not. I tend to think that anything with a lifetime guarantee has so many stipulations that the guarantee is worthless! For instance, they will replace it, but you have to pay the shipping ... which will cost as much as a new pond.
Good luck, though!