That general rule applies to PCs. And I don't know about you but my mariner beastmaster was able to see hiding foes underwater. So who knows.

I think exceptions to this rule would be any of the races that can get that edge that allows assassins and thieves to hide in caves and underground. Orcs might also be the exception. I don't think rangers can see them when they hide in caves, swamps, or their village. But thieves and assassins can because of detect hidden.

When it comes to mobs detect hidden will always allow you to see a hiding mob.