In Chicken Invaders 4: Ultimate Omelette, one type of powerup is a key with glowing golden aura. What does it do?
I've eventually found the answer myself on wikipedia
"...The weapons from Revenge of the Yolk are available as "unlockables". Ultimate Omelette unlockables can be purchased individually by collecting keys, which are dropped from destroyed enemies during the course of the game...."
To expand on that, it seems that those unlockables are graphical upgrades.
To get to the unlockables screen, you needs to click the "my career" button before starting a game and from there on the unlockables button.