Way to fix a leak in the pressurized side of a pool system without digging it up

Hire a water tank truck. Some pool supply stores have them; or depending on the state of your community you might be able to strike a deal with the fire department, subject to no fires happening at the time. Park the (empty) truck, pump the water into it, fix the outlet, pump the water back into the pool. The fire department may be able to treat it as a training exercise; in other cases they are simply happy to help, and in yet others they will not help; it varies with the community. In several places I have lived they are the default pool fill service, and it also means they know where to grab a few thousand gallons of water in a hurry when there are no fire hydrants in the area...

Depending on how good you are at breath-hold diving or whether you know someone with scuba gear, you might also try applying your appropriate type of grease around the outlet from inside the pool. Taping outside seems unlikely to work, IME.

Thinking it through a bit further, (if it's a bottom drain) find/buy a small innertube. Take a 5-7 gallon (20-30l) bucket and try to find a tube just a bit smaller than the bottom of the bucket (assuming the bucket is larger than the drain - otherwise you'll need to upscale the bucket.) To reduce hilarity, remove the valve-stem and fill the tube with water, then replace the valve-stem. Put rocks or bricks in the bucket. Place the innertube around the drain and the bucket on top of it. Drain some water and see if it seals.

A drop of 24 inches in the water level is a definite indication that your plumbing or the pool shell is compromised.

First step is to notice if the water level continues to drop while the pump is off. If it does continue to drop the leak is most likely in the pool shell or the suction side of the pump. If it is on the suction line you should see evidence of this at the pre-filter near the pump. It will be pulling air and water rather than water. Your filter tank when you open the bleeder valve will exhaust air for a long time.

If the water level has stopped and is holding steady you should concentrate your inspection for openings in the plaster shell at this height.

If the leak is on the pressure side of the pump or the return line (which it seems a higher percentage occur) there should be evidence of its location: A sink hole or saturated ground soil. A large amount of your pool pipes are only 10-14 inches below the concrete deck. The only pipes deeper are the drains at the bottom. If the return line is suspect you should be noticing a severe decrease in pressure at the return ports in the pool.

I believe your pool shell isn't the cause of such a serious leak. If it was I would think the opening would be obvious. A drastic loss of water is most likely a cracked or split PVC pipe. Locating it is very difficult for a home owner, but seeing how the professionals failed it maybe time to consider other options: have any of the leak contractors used audio sensing devices? With this type of leak locator they will use a sensitive microphone and listen for the sound of water leaking under high pressure. The work usually entails plugging select pool ports and/or putting suspect pipe sections under air pressure.

Look for secondary signs/evidence of were the leak is originating from. Is the concrete deck developing cracks or are control joints widening? Cracks on the deck may mean pressurized water is eroding the earth the deck is supported on. Is there an area of vegetation that seems to be more lush and growing faster than surrounding plants?

If you are residing in a climate that is prone to heavy rains and high water tables keeping the pool shell weighted down is necessary. If not it would be wise not to saturate the immediate area on the outside of the shell with any more water (especially if you live on the SW coast of N.A.