You thought you found everything? Well guess again!
At the end of every level, after you show Oak your pictures, after completing the game,
(snapping a picture of Mew)
Oak will say "The Report is complete, isn't it, (username)?"
This is an obvious hint that there is more to the game.

After beating all the challenge scores, it's time to unlock the final secrets of this game.

First, more proof that there are more Pokemon to be found in game.

The two levels I hypothesize to contain hidden Pokemon :
-River
&
-Valley