All Secrets
First secret: Level #002
Before throwing your first stars/electrons climb with the little green soldier on the pipelines and reach to the beginning of the game.
Second secret: Level #003
Behind of the red sphere that you have to destroy at the end of the level, there's the secret below a rock.
Second secret: Level #006
Over the top of the central mountain there's the last secret.
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2754981313