I chased a leak last Spring in mine that took a couple weeks of fussing and experimenting to find. There were 3 tiny slits in the deep end where the liner made the transition from bottom to wall. No idea how they got there. Was losing about 50 GPH (yes...50 gal per hour). Wound up using dye. Rigged a thin piece of auto vacuum hose to the end of a 10' PVC pipe and hooked the hose to a garden sprayer loaded with red dye. Started along the seams, then at the transitions. Have to go VERY slowly and squirt just a little at a time. When I got within a few inches of the leak it was obvious. The dye was a solution of water and dissolved stream tracing tablets. Can also get dye markers from a dive shop and make a solution. You'll need a bunch of dye.
After fussing for many days I decided to find what I can see. Started a siphon with a garden hose at the bottom and set the flow to the leak rate. Then played with various dye injection methods to see what I should be looking for. Settled on the thin hose, pole, and garden sprayer.
Leading up to this I measured the loss accurately using a piece of old steel rule ty-wrapped to a pole to get an accurate depth reading. Can also measure relative to a skimmer opening or any other fixed object. Topped the pool and measured the depth every couple hours and let it leak down about a foot. Since the rate wasn't changing that indicated the leak was probably deep. If it had been near the surface then the rate would have decreased as the depth to leak decreased. Once I realized it was deep I then started the dye experiments.
If necessary I can also give you tips on how to lay a patch at the deep end when the water is too cold to get in. That was tricky too. Also, there are electronic leak detectors and I looked into them but decided at about $1400 I'd try brute force first. Also tried making my own electronic version but didn't quite get there.
Hope this helps.
Al