Look for the Lambda sign (the half-life symbol) this is a clue to some hidden goodies.
Also look out for the G-man, you will see him from time to time on your journey.
whenever there is a breencast up, turn on console and sv_cheats 1, then noclip and find a box that when noclipped into, has a dr breen half body (probably referring to half life) - This is not referring to Half - Life but is how the models animation is done. Its way of making the model size smaller, and so there are two models for Dr Breen, one being him standing and the other having the animations in this half-body. This is done for other models,
if you quicksave when ur gonna jump in train then load then you will see a discarted sound "a hint made by valve" it isnt discarted because its in nova prospekt but hard to hear its combine wall ambient sounds.