The "sensor" is that the back pump inlet is higher in the tank than the front, and so stops working to give you a warning before leaving you with an opaque windshield. That's common in other cars I've owned.
Do you have a source for this? NBC? I think you are correct on the actions of the rear and front washer but for the wrong reason.
There is one pickup tube in the washer tank feeding a bi-directional pump controlled by the BCM. However, there is a level sensor. My guess is that when the level triggers the sensor, the BCM may not activate the rear washer but only the front washer. I seem to recall a few years ago, my rear washer didn't work but the front did. I thought the rear might be clogged / line leaking, then checked the service manual, added fluid, began troubleshooting, and then the rear srarted working. I'm trying to remember whether I ever got a low washer reservoir message.
Anyone ever get a low washer message?
Edited by Plus 3 Golfer, 15 October 2017 - 07:51 PM.