Not encountered anyone else yet so don't know the answer to this, but has anyone tried it, or in normal space (and not near any stellar bodies) are you effectively invisible to others?
No you don't, but you do leave a 'wake' -a white maker on the scanner- that other pilots can use for a limited period of time, to enter into your space.
After the wake closes, yes, you become invisible. I've had NPC's appear at random locations tho.
