I've been searching the answer to my question wherever it is possible, but I haven't managed to get the strict rule (or guideline) for it.
As we know the negative form of the past participle is created by the addition of not (Heard - not heard; caught - not caught), but also in the English language there is one other way of creating the negation of such Participles, by dint of attaching prefixes to them, mainly it is Un- (Unheard, uncaught, unseen). The latter, with Un-, work as adjectives.
Earlier I was quite sure that Not + participle is used in long clauses having additional objects, whereas Prefix + participle by itself.
For you to have better understanding, there are some examples.
1- We complained but as usual our voices went unheard.(Adjective)
2- The robber, yet not caught by the police, is said to be armed and particularly dangerous. (Participle clause).
However, there are a couple of cases I'm completely in doubt with. Here they are, below.
3 Our Milky Way galaxy is strewn with billions of planets, alien worlds still unseen by human eyes - at least for now.
(This one I found on YouGlish, the phrase is said by a native speaker, but Why unseen, I think it should be not seen - still not seen by human eyes)
4 I managed to sneak into the hall unnoticed by the security / not noticed by the security
(I think the second one is correct, but on the analogy of the first one I'm not sure.)
5 In the world there are a lot of wonderful places still untouched and unseen by humans / still not touched or seen by humans.
(Personally, I'd write the second, but, as I said earlier, I have no guideline to answer for certain.)
Hope you'll be able to help me, If there is needed to be any clarification from my side, just let me know.
P.S: Before posting the question I'd read all possible sources however connected to relustative adjectives (they are related to the post in some way). I was baffled mainly because in my mother tongue there is a clear rule for that particular case, and if I wrote an adjective instead of a participle in such sentences, it would be considered an outrage mistake. As we all know, languages are different and it is stupid to apply the same rules to every one of them. Nonetheless, I am not alone, there is at least one more persone having trouble here; take a look if you will link. To tell the truth, I'm quite sure that I'll fail any thorough test on this very theme as the vagueness is none the less. It is very strange that there's no strict rule for this part of grammar.