What do people think about this: have ships be invisible to the basic (current) radar if they are "running silent"... that is, the engines are off and they are perfectly still and not firing.

Just a thought...

Maybe not perfectly invisible, but maybe be detected at only 80% of the normal range or something...

Of course, this would also require something like the 'passive mode sensors' that I've mentioned elsewhere. If you are emmitting radar (type) signals you show up really brightly on radar whether you are sitting still or not.