I have 3 Editor Tabs open (Tab#1, Tab#2 and Tab#3 in that order and with identical content for testing) and have Tab#1 as active.
In the "Find" dialog window, I have selected "Scope->All open documents" and the "Find All" button.
Then I get results from all 3 open documents in the Message window as expected.
But If I have Tab#2 active, I only get results from Tab#2 and Tab#3. And if I set Tab#3 active, only results from Tab#3 is displayed.