How can I locate an inground pool leak?

I have a 16,000 gallon inground pool with a fiberglass finish over a gunnite shell. It has a 8-pop up infloor system which has been disabled, so the popups are the only returns. I have a leak somewhere, without the pump running, but I can’t seem to find out where.

I plugged both holes (cleaner and main drain) in the poolside skimmer, and the pool is still leaking at a rate of about an 2 inches a day or more. I patched the hole at the pool light conduit, where the power cord enters the pool (in the niche behind the metal pool light assembly, and replaced the gasket on the pool light assembly. Still leaking.

So, I guess that leaves either a main drain leak, Pressure-side pipe leak, or one or more of the infloor pop ups? Squirted some dye around 3 of the pop ups and saw no movement. Any help would be most appreciated. Thanks.

A Quick Update

I drained the pool and inspected the bottom/main ‘drain’… which is NOT leaking. I dug up the suction-side PVC from the pool to the pump, and noticed a small drip on the bottom of the PVC at a previous repair connection. But it doesn’t trip when the pump is running, and it seems too small a leak to be dropping the pool level by an inch (or two) per day. I continue to suspect the leak is coming from one, or more, of the 8 (disabled) pop-up/return ports on the bottom of the pool. Or, at the T connector under the concrete at the pool edge where it hooks up to the suction line that goes to the pump. The pool is an old Master Pool, and I can’t find anything online about the pop-up caps or how to remove them. One photo shows the pool level when the leak appears to slow a bit.