Finding a leak can be a challenge... as Hubby and I recently found out. If you're certain it's not the plumbing, that's a good start.
I did the whole food coloring dye thing and found nothing. We have lots of little dimples and divits in the bottom of our liner and I just couldn't cover them all at one time. Hubby did a night swim with a flashlight in a ziplock baggie (we don't have a light in the pool) because I've read some say that the light reflects the water differently at night.... I thought we had only a few dimples in our liner before that... boy they show up like crazy with a light at night! We didn't find the leak that way either. I was ready to go around with the dye with a more methodical approach, but that never happened. We were swimming one afternoon with some family over... Hubby and I had been keeping our eye out anytime we were swimming because the leak had begun to get worse. While he was doing laps back and forth across the pool he jumped up and exclaimed, "I found it!!!" He wasn't looking specifically for it at the time, but just happened to catch his eye as he swam.
I would start by checking all of the seams. Walk around in your pool and see if you feel any spots that have a deeper dimple/divit or feel soft when you step on them. Swim along the bottom and run your hands along as you swim. You might happen to feel the area that's damaged. I had my son stand on my back while I swam along the bottom because it's difficult to swim slowly and keep yourself down.
Our hole was more like a slit in the liner maybe 1/2 inch long. We were losing lots of water and couldn't figure out where it was going because there was no evidence of the extra moisture outside of the pool edges. We were pretty certain it wasn't a wall leak because of that.