Update: the water loss with the pump off overnight and on today was minimal. I reinstalled the eyeball and lost 1/2" of water in 1 hour. When I installed the eyeball, the pump pressure went up 3 or 4 psi.

So, after looking at some potentially related posts. I'm wondering if I could have a cracked pipe. When I install the eyeball and the pressure increases, then the crack opens up wider and leaks more water. I remove the eyeball, the pressure drops enough for the crack to close, and I'm not leaking enough to notice.
I'm assuming it's gotta be on the return side

Is this a possibility? If so - how in blazes to I find the crack?????

With this behavior, and no air in the lines, I'm assuming it must be on the return side. Right??

Additional info: 24' x 13' I-G concrete pool w/ sand filter