For some reason, if I fire swarms then jump somewhere else, the trails are rendered there too. Not sure if anyone else can see them, or if it's just me.

System Specs:
Ubuntu Linux 7.04
Athlon XP 2500+
1 GiB RAM
GeForce 5600 256MB
IBM Model M keyboard - the clicky kind

This sounds similar to the graphical bug where the engine trails of the first ship you dock with (per session) are visible in any other sector afterwards at the same x/y/z coordinates that they were when you docked (no, I don't know how to get the actual coordinates, but it makes more sense than them just randomly appearing...).

It's your keyboard's fault. It's too loud. If you get a quieter one, you wouldn't have that problem.
Seriously, though, the trails aren't properly cleaned up between sector loads. That's on my to-do list.

[X] Ray

And Ray's first answer FTW!!!