If they've set their security settings to friends only, then you won't be able to read their posts.
Btw, although they don't get a message/klaxon alert if you've nosed on their page, you will probably turn up on their 'people you may know' feed. I know this because two people turned up on mine who I know, but who have absolutely no connection with anybody on my Facebook friends list. Therefore I can only conclude that they were nosying!