There should have something to tell that you searched the whole building like a pop-up, Like when i was at a gigantic hospital and i had to search the whole freaken building with the mouse, That was annoying.
guys! found a secret boss! where to find it: he is on the snake pit and you have to find his secret room once you find the room stand on the button and he will apear... one more thing, he drop tier 5 and 6 itens. click at the + if this helped you out