In Spy Mouse there are secert and hidden levels. To unlock them, you have to go to the backpack, then to worlds, then you have compare the preview of the worlds with the worlds. If you look hard enough, you will find levels in the previews that are not in the worlds. Then, you go to the level that was on the path to the hidden level (in the preview). Finally, you find the crack(s) in that level. Go in it and find another crack, from that crack, you will go to the golden door.
In world one, there is a secret level. In order to get to it, you just make a path from level 1-7 to a piece of tar diagonal from it. In world two, there is also a secret level. To get to it you make a path from the boss level to a pile of trash, located straight past the "Play!" sign.