Is there a good time to use 'może' over 'chyba'?
I would say that "może" expresses wondering about some subject, whereas "chyba" is used when you are not completely sure about validity of information which you give.
Czemu nie moźemy powiedzieć: "moze być on jest policjantem?"
That's a calque from Russian. Polish, if anything, will use "być może".
On moze jest policjantem. didn't accept this answer
That doesn't seem correct to me. I think a grammatical way would be "On może być policjantem", but that's "He may be a policeman".
Gliny są wszędzie!