Dear R. de Boer,
After reading it a few times, I have made an analysis of it.
Recovered post-rinse solutions shall be monitored for a build-up of carry-over from detergent tanks.
Build-up = Increase of concentration over the time.
Carry-over = Contamination by means of the carry-over principle (with detergent in this case).
The term "carry-over" is normally used for additives. It means that an additive may come together with an ingredient. Simple explanation: An additive which has a function in the ingredient, but not in he finished product.
By post-rinsing a tank, one removes all the detergent rests and by reusing this rinsing water, there may be traces of detergents in it. And this last thing has to be monitored, to avoid that there are detergent rests going into your product. (There has been a case, I am not remembering exactly, a few years ago, with ice-cream).
I hope this can help you further. Maybe other members can be helpful to give more practical information on how they handle this. However, there are indicators to easily detect detergent rests.