Any Mopar Tech's here - Grand cherokee 4.7 loosing coolant.

In my opinion you need to do a cooling system pressure test (with the vehicle running and up to temp with a few revs while watching the gauge on the tester)
This will tell if it's getting compression into the cooling system, if the person doing the test knows how to do it and knows what they are seeing.

If you are positive there are no leaks then I'd bet that's what is going on.
Something is pushing the coolant out and then sucking the recovery dry when it cools down again.

Wouldn't be a bad idea to put a new cap on it in any case, as that can be a cheap fix for these kinds of problems as well.