I nearly had a panic attack when you guessed this - it very was nearly a legitimate alternative answer to mine! I can tell you thought hard about this, but I've spotted one or two flaws:
In one of my forms, you will see me and recognise me. In another of my forms, you will see me without recognition. I am unknown in another of my forms.
Psymon:
-We can detect 14C with radiotechniques,
OK, so you did something sciencey here

. I'm going to be picky and say that although you can detect Carbon using radiotechniques, you cannot really see it. Also, this is not really recognition.
Psymon:
we know 12C is everywhere even when we cannot individually recognise it
This doesn't really make sense either, as you are not answering the specific clue of being able to "see" it (you say "we know it's everywhere" ). Also, you said "not individually recognise it", which should be "not individually see it". "See" and "Recognise" are not synonyms. Therefore you are saying that you cannot see 12C as an answer to "you will see me...".
Psymon:
21C is an isotope with a mean life shorter than 30 ns, so I assume that it's never been seen, so unknown
Now, just because it has never been seen does not make it unknown. The fact that you know of 21C disproves your comment!
This was a good answer but I think you should agree that your explanation for this was not perfect: "In one of my forms, you will see me and recognise me. In another of my forms, you will see me without recognition. I am unknown in another of my forms.".