My experience of having explored a lot of random planets and moons, is the scanner will show place compass points for all POIs up to around 1.2km.
The planets generate as you land at a point (Todd said this in June's Direct), then then the POIs generate around you explore. From a high point you can see POIs beyond visible scanner range, what I'm saying is, if you are on one side of a mountain/hill, there is little point exploring it for caves or recessed bases because if there was something present, it would already be visible on your scanner.
Right, and I think Bethesda did a fantastic job here. Not only is the procedurally-generated topography diverse, believable and in many cases, absolutely stunning, it's clear that they invested in ensuring that the way resources are seeded naturally on planets and moons is semi-scientifically sensible, or at least consistent. Where I think Starfield misses a trick is you don't need outposts at all. I discovered that between opportunistic mining, scavenging and occasionally buying resources, was all I needed to craft weapon and suit mods.
There was a really
interesting post on reddit yesterday, which I would recommend spending five minutes to read. It speculates that the mid-development change to fuel consumption (to fuel not being consumed on jumps) rendered the outpost mechanic as highly optional and I think his person hit the nail on the head, particularly when you consider the UI retains elements of fuel consumption being a key mechanic. If He3 (fuel) depleted you explored, you would need He3 refuelling point around the Settled Systems to explore further out. This would mean building Outposts to produce He3, and ship to other outposts that existed as refuelling points - and to defend those outposts as well.
There looks to be a ton of depth to the outpost building, but no need for it from a gameplay perspective. I think fuel consumption will very likely return in the upcoming survival mode and like the survival mode in Fallout 4, it will massively change the game. Just like you could ignore settlement building in Fallout 4 main mode, in survival mode settlements become necessary to support exploration around the commonwealth. When you cannot infinitely fast travel, you would need outposts as points to rest, restock, refuel and offload cargo to ship it back to Jemison or Akila to sell.
There are different 'size' POIs, some come in small, medium and large variations with the medium size ones seemingly generated the least often. This may be what you think is a unique POI, that or your stumbled on a unique structure tied to a main quest but outside of the quest itself. Continue exploring and you'll see that total number of POIs designs are under 20. I think it's 18, the layouts have been found in the game files.
If the game can procedurally generate unique bases, it's not doing so at launch.