Can you say maltrovis instead of kasxis?
I would say that maltrovi = perdi (opposite of find = lose) rather than kaŝi (hide).
Ah, of course, that makes sense. Have a lingot.
Mi kaŝis ŝercon sur tiu ĉi paĝo.
if you say Kio vi kasxis?, (not kion) what does it mean?
It means nothing.
It's like asking, what does "He see I." mean in English. It's grammatically incorrect.
People would probably understand you (the Esperanto as "What did you hide?", the English as "He sees me"), but they're still wrong.