My fleet just suddenly disappeared, while travelling through a system (Olvaldi) - I uploaded two screenshots so you can see what I'm talking about.

Sucks pretty bad since its my primary Battle Fleet and it was on route to attack my annoying Craver neighbours :)  

As far as I can tell it must be a bug - The fleet doesn't show on the map at all, but according to the Military overview its suppose to be there in that system...

Anyway I just solved it be reloading an autosave, but still - Strange thing, that must have been a bug..