why does one not see it at the edges of the field, or where the soil is supposed to flow to?

Appears to me to be no different than a slight rain on a field------when the water runs to the edges and the ditches, becomes noticeably much more.

There has to be some kind of accumulation somewhere if that kind of loss is actually occuring.

I can see "slippage", ie a changing of location a few inches.........but actually leaving the field---------- has to be some kind of physical evidence.