Con't...........

Something else:

If in front of your pump there is more then one line, and a valve is closed (and always has been) then I highly suspect this is the main drain line. A plug was placed in the main drain line to prevent water from seeping out, and the valve was closed to prevent the pump from drawing air (as described above).

Your initial thought about the drain are spot on.

Anything additional under the ground, out of your control (laterally plumbed and not individually) can be a nightmare.

If you have any questions, it might be helpful to look over what you have first and describe the pool as best you can. IE. how many skimmers, valves, lines, returns, "holes in the bottom of skimmers" etc etc.

Just what your think is pertinent to a follow up question.

Good luck....