Thrust signature
Every time a new one of these comes out I hope the SETI folks are looking at the exhaust signature to see if they can identify it in their sky observations. That would give us a clue about if anybody else out there is using it, and let us know if they're decelerating on their way here.
Sooner or later they're going to figure out how to make fusion work to power the thing, and Hydrogen plasma thruster, and ion scoop fuel collection. After that we're off to the stars.