Not really much of a mystery. I've had the pond fill-line shut off today so I could patch on the waterfall some more, and I thought that the drain-line was off also, but all day I have watched the water level go down. How could that happen? Just a few minutes ago I walked to the dumpster and noticed that my sister has a sprinkler going from the drain-line. The missing water - mystery solved.
More repair work done on the waterfall and will know tomorrow how well that works out.
Under 100 degrees today and that has been nice.