By hippo effect I assume that means floating liner? Hadn't heard that term before. If you do mean floating liner then no, a leak will not always produce a floating liner. It depends on the elevation of the pond's top, soil type and a few other things. Also you would have had to be present to see the floating, meaning maybe it did float for a time. Normally leaks are slow enough not to cause a floating liner. Normally it is caused by rain water draining into a low sitting pond and the water getting behind the liner.
And the floating liner thing can present as a leak like you described. Not saying that's what the issue is, I don't know many details. But if it rained before you noticed the drop I'd be considering that being the cause. Leaking sprinklers can cause it, heavy watering, etc... Rain water, and may not take much at all, gets behind the liner and rises the water level above the liner top and the liner has no downward pressure so just floats. When the rain stops the water drains into the soil and the liner goes back into its normal position. Owner comes out and sees the pond down a foot or two and assumes a leak. Again, not saying that's what happened, just possible. It's pretty unusual to get such a big leak suddenly in a mid water column location. But I know nothing about this pond.