I had a pressure side leak a couple of years ago, and ended up hiring a professional lead detector to come out and find it. I think the cahrge was around $350, but he did pinpoint the leak which was in an elbow fitting under the concrete at the corner at the shallow end of the pool. (the pool builders had used a drain pipe elbow fitting, not a pressure pipe fitting, which has a smaller mating surface, it blew out after 29 years).
He first used a detection device that looked like a heavy coffee can with headphones, he said this was good for very small leaks, but not so good on larger ones. After spending about half an hour not finding anything with this device he then had me turn off the pump, plugged the second return eyeball with a custom built plug, and screwwed a custom built adapter into the remaining eyeball fitting. This adapter allowed him to connect a standard garden hose to the eyeball and back feed it. Making sure the garden hose was full of air he connected it and turned it only slighlty. This forced air into the lines and he went around with his ear to the ground until he found the gurgling hiss of the leak.
Next time I have a problem I think I will try building a plug and a hose adapter out of plumbing parts and trying it myself, i suspect it could be done with $15 or so in parts. My pool has a single return line that feeds 2 eyeballs, therfore the need to plug one.
Ike
p.s. my leak was around the size of the one you describe, maybe a bit more