What are the things that separate cultures here on earth, in a purely abstract sense? A barrier of some sort, of course. An ocean, a desert, a mountian range. These are barriers the hardy and the lucky might traverse, and leave no sign of their passing.

Apply to space.

Maybe it's not a point to point journey. Perhaps it takes place as a series of "jumps" through "difficult" areas of space. Very much like species have been doing on this planet for a long time. It's natural, actually. And luck has a lot to do with it, so does serendipity, and brains.

When the few humans landed on Easter Island, they were essentially lost to the rest of the world, and none could possibly track them. Just the vastness of space is a good place to get lost and we know that the galaxy is not homogenous: it has it's own "geography". For example, we are out here on the outer spiral arm, which would make us pretty remote already. The star void space between the arms and the main body of the galaxy would be like a "desert" in galactic geography.

Another thing...once powered acceleration was completed, would a craft at speed leave a wake of any kind through a vacuum? (excluding constant propulsion systems like ion...) Of course, you've got to turn around and slow-down sometime. Which complicates the "oh, let's stop and have a look see at that planet" storyline. You kinda have to have a flight plan and start slowing down long before you get to where you want to go. Or do you guess and get lucky?

Sometimes, settlements in the western United States came about because that's where their oxen died, or their waggon broke, or a baby was born, or they just got fed up with travelling and settled in the first decent land they could find. They gave up the dream of the land of milk and honey and took what they could get, 'cause they had to.

Crafts get lost in storms, too.

I don't know if any of this could possibly help, I realize I'm rambling... Enjoyed what you've posted so far.

jms