Ok, so.. back to a known location to get some knowledge on the limits of the 'barnacle visible distance'. Approached a known location just before dusk, and manage to get pictures from a distinguishable 'POI' from up to 4.45km, and this is a destroyed barnacle.
Any higher than that and the spot disappears. This is considering a straight line of view, so to be safe, I'm setting my maximum hunting altitude to 3km (giving me a span of another 3km view to cover the whole canyon width). That is by far much better than the current 800 meters I was using and got my hull down to 35% after a few oopsies.
There no difference between a intact and a depleted Barnacle from that distance and the only difference seems to be the 'sparks' reflection floating some meters on top of the barny (much more noticeable in movement):
Nothing seems visible from SC. Even if SC can be activated at 2.5km (masslock distance) it is still the escape vector SC and by the time you get into orbital flight it is too far.
Enough information collected, heading back to IC 4604 again, main focus now are small (400km) high metal content potatoes almost no times earth masses (0.0010 max). Cheers!!!