Flyrodde;
So far, it's ONLY been found in the dog food. Nothing has been found wrong with the distributed cat food. (yet).
And, yes, there IS an additive that can be added to dog food, to help break a dog of this "odd habit". It works. We used it to train several dogs, of ours, that had developed the "taste treat habit" when younger. (Sorry, it's been a long while, since we needed to use it, so I can't remember the name off hand!).
It was expensive, I know! So, we ended up using straight table pepper, sprinkled on "training piles", which ended up breaking the habit every bit as well as the expensive additive had!
Backcastwards; Your "humbly rubish" actually proves the fact of a canine's mouth, versus a human's................ in point, if YOU ate what you're describing THEM eating.......imagine how long you'd last? With the antibodies, that are found in a canines mouth, however, you notice that THEY have NO ill effects from their occasional "taste treats"!?!
And, I totally agree, with you about......... "And, no, they are not underfed", (as a reason for why some dogs enjoy eating pre-owned food from a another dog).
"Not having enough to eat", "They're missing something in their diet", "They are instincivly replacing needed vitamens and chemicals in their system"................ etc. etc. etc. ARE ALL nothing more than "speculation", mixed with "wild guesses" and a few "old wive's tales".
After many, many, years of studying this strange canine behavior, no one has ever been able to solve this "riddle". Some dogs, (not all), like to eat "lawn grenades", and, SOME dogs do not. No one, however, knows "why" for sure.
With our own, kennel dogs, we would have a pair of pups, like two brothers, from the same litter-born and raised here together-fed the exact same foods-housed together-everything exactly the same.
ONE would soon develope the lovely habit, the other brother wouldn't go withing 5 feet of another dog's "deposit"!?!
Saint Paul-"The Highly Confused"
You cannot do a kindness too soon, for you never know how soon it will be too late.
-- Ralph Waldo Emerson