In order to fully explore a system...

[varonica]

Thankx again. So Guardian sites are to be found mostly or always on rocky worlds?

The crashed ship like the Hyfords Cache Type 9, appear only if close enough to the planet, but yes there is a "human" signal on the body while there is actually no settlement that it would referr to otherwise

That's the human signal, it will tell you there's human tech there, but you need to get to the planet identify and locate it!

As for exactly what for type guardian ruins appear on, Canonn reseach has a full list of them all and can probably easily tell you, I only go personal experience and the ones I have visited have been on rocky bodies, but that's not necessarily definitive, just personal experience.