Don't panic. If you are pretty sure that the level drops faster with the pump on, you may have a clue. Once the level drops below the skimmer you can rule that part of the system out. If the sanitizer level and ph stays where it needs to be the kids can still use the pool safely. The next thing to consider is the multiport valve on the filter. Quite often the valve that controls the filter will leak internally and send a constant stream of water to your backwash discharge. If this terminates in the yard or into an open drain this is easy to check. Sometimes they are plumbed directly to a sewer and you will have to open the line somewhere to see if water is flowing to the sewer when the pump is on. Once you eliminate this as the source, try plugging the return inlets and valving off the skimmer,vacuum, and main drain. Let that stay plugged overnight and check the results. Cracks in the pipe or separations at joints many times do not leak sufficiently without pressure to show up with the die test. Professional leak detectors will seal off the piping and apply a small amount of pressure, if it holds pressure, no leaks. If you try this method be very carefull as the system was not designed to handle large amounts of pressure and you can easily create a problem that did not previously exist. Keep looking, you will find it.