If you've downed a historical ace who cant die, have a message (pair of boots and a note dropped on your airfield) that you downed this ace, but he lives (since he cant die) and he's looking to repay the favor (and some kind of AI behavior that makes said ace target you if he sees you in a fight).

Since squadrons square off vs. each other all the time in the same area, the likely hood of seeing the same ace is quite probable.