Ok this is interesting,
I was attacked by a bigger fleet, so I hit the try to disengage option to be able to attack the enemy more spread out then all at once.
I easily defeated their 2 carriers and lots of fighters, which made part of the enemy fleet retreat, but it did not say that they were defeated nor did it give me the option to end the battle.
Looking around the entire combat map, I couldn't find any enemy craft but I could only 'Quit to main menu' so I decided to retreat in the hopes that would end the battle.
While retreating I found two fighter wings between 36000 and 54000 meters above the combat map. Unable to engage them, I had to retreat almost making me lose my fleet...
TL;DR
Fighters (and possibly other craft) can get outside the combat map
P.S. I'm unable to provide a save as that save was a long time ago...