OK, I had a chance to test the large military fortification, the one with the huge exterior walls.
Managed to get my ship within 600m of the base in a low gully that I thought would protect me; lowering the SRV, the contacts were all yellow. However, as soon as I started driving it slowly toward the base (still under cover of the gully), the contacts turned red and they were alerted to my presence, still within meters of my ship.
So I am guessing that it's not only line of sight, but also a minimum range at which it doesn't matter how stealthy you are, a ship has loud engines and an SRV has slightly quieter ones, and you'll alert the guards/scavengers one way or another.
For the moment I am finding that around 400m is the safe point for disembarking the SRV, and that about 1.5km is the ship's safe distance in order to steal into ta settlement and take to the rooftops for some healthy covert sniping action. I think you can get much closer with the ship, still need to do my 'leap frog' test for that one.
You can probably get closer than that, although I don't know what those numbers are.
Although it's a big change from before, I feel it's more reflective of a 'real' situation (scaled down for game play purposes) and adds a sense of realism to the game. Previously I could float above the settlement in my ship with my lights on looking closely at the base layout, buildings and streets etc.; now, there's none of that and you've got to sneak your way in.
Just hoping they fix the flashlight problem!
Also: how do Apex travelers deal with this situation? Do the guards/scavengers go nuts when you disembark from the taxi?
Finally: if you do make a mistake and alert others to your presence, simply SCing away and back does fix it (for the moment...)