Has the secret final level got an orb or it's just something that was left there? If there is an orb, does it imply beating the boss without getting damage?
Great fun game, btw!
If Drake used the orbs to get somewhere secret, then there isn't any extra orbs to collect past that point. The map just always shows an orb slot for each level. There isn't any secrets to the super secret boss besides getting the true ending.
Really glad to hear you enjoyed playing it! Someday I'd love to make more Drake, sadly life hasn't given me the time since work has been crazy. I do have plans for another adventure, though a little (but not much) more hi-res! ;)
Still, it would have been fun if you could have get an special thing for beating the secret boss unharmed. Also it would've been nice if prisoners also gave you something similar to the orbs to get more replaybility. Great fun game!