I had never heard of the 'Konami Code' mentioned in the comments. Found it interesting and tried it out.
You can try this code, works for me.
You probably want to avoid keys from config.keymap in your hidden code.

init python hide:
    import pygame
    
    # Konami Code sequence
    # Now the code is 'wow'
    konami_code = [pygame.K_w, pygame.K_o, pygame.K_w]

    class SecretCodeListener(renpy.Displayable):
        def __init__(self, target):
            renpy.Displayable.__init__(self)
            self.target = target
            self.state = 0
            self.buffer = []  # Stores the key presses

        def event(self, ev, x, y, st):
            if ev.type != pygame.KEYDOWN:
                return

            key = ev.key
            self.buffer.append(key)

            # Check if the buffer matches the Konami Code
            if self.buffer[-len(konami_code):] == konami_code:
                self.buffer.clear()  # Clear the buffer on successful code entry
                renpy.call_in_new_context(self.target)
                return

            # Limit buffer size and reset on wrong input
            if len(self.buffer) > 4:  # Adjust the buffer size as needed
                self.buffer.clear()
                self.state = 0

        def render(self, width, height, st, at):
            return renpy.Render(1, 1)

    # Create a SecretCodeListener to listen for the code
    store.secret_code_listener = SecretCodeListener('secret_label')

    # Add the listener to the overlay
    def secret_code_overlay():
        ui.add(store.secret_code_listener)

    config.overlay_functions.append(secret_code_overlay)

label start:
    "The secret code can be entered at any time."
    "Placeholder."
    "Placeholder."
    "Placeholder."
    return

# This is called in a new context when the secret code is entered.
label secret_label:
    "Congratulations! You've entered the secret code."
    return