It is unknown where the cat is hiding. Could this also be correct?
i also type that but it seems not corret
Yes, that would be correct. Hopefully it is accepted by now, but if not, it should be reported so they know.
I'm not sure I understand the circonstances where the passive tense is used. (haha). That sentence, for instance, would be used to say "we don't know where the cat is hiding" or " one doesn't know [...]" ?