I haven't found another gun. Have you found the binoculars? They're on a little daias above the pool (where you pull down that ball to lower the plant pot). Oh and, the last three secrets (i think you mean the ones with the clues) You have to get them in order.
If you don't have most of the others first, they wont appear either: Turning the statues above the pool can only open the underwater gate, not the clue stone.
I don't know: I did all this after i did the Peru Level. Maybe you have to do that first too?