There's a few things to watch out for... At each station, there will be questions, puzzles, riddles or things for you to find and solve. Those reveal numbers or letters, that you will need at the end of your mission, so note down your answers and their order very carefully...
Additionally, starting at your next stop, you should take a close look at the path you walk... you will find some arrows, that are occasionally followed by some numbers. Note those down SEPARATELY from your password symbols that you find.