Sgarfofalo:
Think about it logically (and I could be wrong). The MD is an opening where water is being pulled from leading back to the pump, which also includes the skimmers. If there is a valve there, it would not be the best design given that your pool could empty. The MD should be buried in the concrete mix (sometimes vermiculite) and then the MD is completely sealed with gaskets, the liner, screws and the cap. Where could a MD possibly pick up ground water is beyond my knowledge. When ground water rises, if enough pressure is behind the liner, you will see wrinkles, bubbles and possibly the liner could float. Some people have the same issue year after year until the ground water settles.
If the plumbing (piping) of the MD was broken, you will not have any water when you get home later. If it is the MD, I am guessing that the gasket(s) where the liner meets along with the screws is the problem and nothing else. This is why I would have not filled up the pool until the leak completely stopped. Once it stops, you know the problem is above the water line. If the pool completely empties, then clearly it is the MD. If this is the case, by you plugging the MD, the leak should have stopped. If the leak has not stopped, then it is either the gasket(s) around the MD or something in the liner near where the water stopped or emptied. Please tell me where the water level is when you get home.
Swimming Pool Hydrostatic Relief Valve - YouTube
This explains that valve well. It is a one way valve and possibly could have been stuck open (maybe)
Hydrostatic Valve
Like any one way valves, they have the possibility of leaking, get stuck, etc. Do you have high water tables where you live? If indeed you have a valve and it is in need of repair, the procedure seems pretty straight forward. I personally would call a professional at this point.
Luckily, I do not and am over 60' higher then the first home on the block and there is only 12 homes in the whole development, making the conditions ideal. I actually took a close look at my MD and they can be plugged in the event one breaks underground (hopefully not).