Revenants can easily spot an SRV from around 600 meters, I believe - regardless of whether it is Scorpion or Scarab. The discussion about ‘stealthy approach’ is more on a (presumed?) bug where Scouts spawn in less than a kilometer above the surface and so detect the Scorpion*.
*Cannot say if this would also result in detection for the Scarab. Supposedly that spawn doesn’t happen with it?
Regardless of that, if you want your SRV within 400m of the settlement, best park it in a ditch with no line of sight somewhere, if available. Otherwise around 700-800m distance, maybe just outside 600, will have to do. In my experience.
Yep, I can confirm this with 30+ of these missions done (though retired now, due to the rarity of these missions).
All SRVs can be spotted 600m away by Revenants, though their detection range is less than that (400m?) if they are still white (not on alert). For extra protection, try to park your SRV out of LOS of the base (behind small hill, behind large rocks, etc.); that cover really helps.
Occasionally you will get "Frameshift Anomaly Detected" warnings, even when it's NOT a scout replenishing Revenants. This appears to be scouts looking for ships still on the surface. If you've made a non-violent, stealthy infiltration and exfiltration, your Scorpion
may not be detected by the scouts. If, however, you've blown up any Revenants, you may find one or more scouts hovering low to the ground. The minute you get in your Scorpion, they go hostile, and actively attack you (and will destroy you if don't take evasive action until your ship arrives). Scarabs, on the other hand, appear to be completely invisible to Thargoid scouts. It's extremely aggravating that this bug exists... more so that it affects the new, military themed (and two seater!) SRV and not the old standard Scarab.
I was actually going to document this after Update 16 dropped. However, they've limited these missions to only being available from Alert Systems and - thanks to the another change in Update 16 - the Alert can be cleared BEFORE the next server tick.
This issue is not logged in the issue tracker because you need to provide lots of evidence - and then keep getting likes on your issues - for a ticket to not expire or be silently closed; that's why there is a thread in these forums to keep track of
popular issues that have been reported, to have people keep confirming them to keep them open and (supposedly) on the Devs radar. As someone who has worked in IT for over 30 years, their issue tracker ticket system is the worst, and stupidest, ticket system I have EVER seen!