Is the grey stuff the thing we make fond out of
Fond isn't just browned protein scum. There really are small bits of meat/fat that get stuck on the pan, and you should be able to see them before they're thoroughly browned/burned if you pay close attention.

That kind of sticking isn't the same as when meat sticks really obviously and badly. When that happens, you'll naturally notice when you try to move the meat, and be aware of what's sticking to the pan. But even without that kind of awful sticking, you can have tiny little pieces that stick, then tear away from the bulk of the meat when you turn it. You won't notice nearly as easily, because most of the meat isn't stuck.

The exact details depend on the type and cut of meat, I suppose, but in general it's quite easy for this to happen.



As for the additional questions... proportions probably vary a lot, but I'm guessing in terms of protein and fat (i.e. without the water that you're just going to cook off) there may actually be a lot more in the stuck bits than in the juices and protein scum. I know that the really good fonds come from pans that have a lot of obvious stuck bits, not just an even coating of cooked-down liquid across the pan. I'm sure you could make something without any stuck bits, but you probably won't be able to make nearly as much as you'd get from the normal process.