We are translating a passive sentence as an impersonal one. Why not use the Esperanto passive participles, which by the way were offered to us in the former module? Could we also say Karbon estas/is trovita apud la vilaĝo?
I would say "Karbo troviĝis..." Use of "one" in English is pretty limited and idiosyncratic (usually only in prescripitives or hypotheticals, virtually never in a non-conditional past tense like this). Is oni much more common in EO?