Search doesn't span multiple editors, and this is confusing but it's surprisingly hard to fix this as the search is tied to the editor control used. However I can't get it to fail as you describe, multiple editors open, also an entity in its editor, selecting a tab, pressing ctrl F, typing a string, it finds it, switching to another tab it still finds it (but you dont' see that).
So it doesn't search on the active 'selected' tab, it searches in the tab you had selected when pressing ctrl-F as it's tied to that tab. We can't make the search dialog modal, as you then can't scroll in the editor tab you had selected.
As the control library we're using has pull out window parts now, we can look into placing the search at the top of the editor pane in the tab, so it's obvious where you're searching in and the confusion would go away. We'll research that in a future version.
