IP leak, logical steps to take to locate. which first, second etc.

I have a 16' x 32' inground pool, fiberglass panel walls, painted concrete bottom, aprox. 25 yrs old. I lose 2-3 inches of water a day. Pool company has been here 4 times. The only thing they did that has helped at all is to replace hydrostatic relief valve in main (bottom) drain. I do not know if they have pressure tested the plumbing, company owner said they would do this on third trip, do not believe the worker did this. They leave no paperwork showing what they do each trip. They did putty or seal around skimmer plate and return jets. They attempted to seal one seam top to bottom between panels. They left 3 or 4 gaps about 1 1/2" long. they checked light twice, worked around skimmer twice and put a small amount of putty on two cracks in steps. Thanks!