Hidden powerup:
If you've played through the game once, you may have noticed that near the end of the game there is a tank that Marta can use to fill herself like a balloon and go unnoticed by the last guard.
Well, there's another one hidden in the early stages. You can do the same with this one, and this way you can go through most of the game guards without being spotted.
Have you found it?
Have fun!