There's this popular treatise on what it means to be hidden called The Rules Of Hidden Club posted on the WotC boards.
It's widely regarded as a good and toroughful explanation on what being hidden means, how to achieve the condition and how not to lose it, and it looks like the author made a great effort to be sure his grasp on the rules he was explaining was solid.
To date, whenever there's a discussion on this site or in a forum I usually write in, you can be sure this article gets linked. I trust this to be a good indicator of the usefulness of said explanation.
The Fifth Rule states: "An enemy can spend a Minor Action to roll Perception"
Page 186 of PHB states that actively rolling perception is a standard action.
You can see for yourself how those sentences are apparently incompatible. What happened?
Is there some difference between PCs and NPCs?
Was there some Updates and Errata or some later manual changed how it works?
Is it just an error by LordOfWeasels?