In darklight's unity test framework video (here https://www.youtube.com/watch?v=kmsFqQlDoeY) he had a dynamically updating indicator showing his current distance down the runway.

I think it would be useful to incorporate this text in the actual game for telling you how close you are to the bubble's edge.

The text should say something like:

You are in the bubble and your vessel is not yet synced. (Distance from Bubble: ##m)

And once you leave the bubble it should say for a moment (3-4 seconds):

 You have left bubble and your vessel is now being synced with the server.

Or something to that effect

Closing this, I think most people learn about the safety bubble relatively quickly, and it's easy to spot when you leave the bubble.

It would be probably more annoying for the players that have already learned about the bubble :P