I've worked for a large company where if you know someone's band (and you generally know that just because you do, not by asking about) and how long someone has been there, you know within about a grand, how much they get paid.

I now work for a place where I'm told to never talk about my salary, yet 5 members of the (very small) company were invited into my salary review.

I often think that the more they stress to not talk about it, the more likely it is I'm underpaid compared to my coworkers.