I know it's unlikely but...hum, an UFO maybe?
I mean, not a flying saucer, a proper ufo. Like a flying orb or a large pinball.
Something that would leave the player wondering what he just saw zipping by his canopy.
Cherry on the cake would be to "hide" it inside another mod, like a "barrel and crates" mod or something so it could be added to a mission without the player suspecting it.