Yesterday, I read a great and somewhat surprising post on the Tao of D&D. It was surprising because it got right to the point and dealt with meaningful issues. Except, of course, it turned out that it wasn't on Tao of D&D... I just mistakenly believed that, for some reason. It was on Hack & Slash. (Sorry for thinking you were Alexis!)
Anyway, one thing that jumped out at me from that post was a distinction near the end between two types of concealment: hidden vs. secret. A hidden door is automatically found if someone searches the area it is in. A secret door is only found on an appropriate roll.
I don't run it quite the same way, but the distinction still stands: I treat things that are secret differently than things that are hidden, and hidden things can be fairly easily found. We all have a pretty good idea what "hidden" means. But what, exactly, makes a door or compartment or other item secret, instead of merely hidden?
For me, it's not that secret things require a die roll. It's that finding them does not necessarily allow you to use them, or even know what they are.
"You find a hidden door." It's behind a tapestry or covered with plaster or something, but once found, it's pretty obvious that it's a door. If there's no knob, it might be difficult to open, but you at least understand the concept.
"You find a secret door." Actually, it might not be a door. What you find is a section of stone or wood paneling or whatever that looks different. If it's a door, you don't know automatically how to open it. It might not be a door, it might be a pressure plate that triggers a trap. Or it might be a secret compartment, Or it might be a sealed doorway/compartment, with no mechanism to open, just a hint that there's something behind it (fetch the sledgehammer...)
Finding a secret door isn't automatic for general searching. The point is that it doesn't look that different from the rest of the wall, floor, ceiling, or other feature that it's part of. That's why there's a die roll. But finding it might be automatic with other methods, depending on the details of the secret. Secret door with a light source on the other side? Dousing all the light sources on *this* side automatically detects it. Windy area on the other side? Candle flame will flicker as it passes near the cracks. Secret compartment in stone wall? Tapping it makes a different sound than tapping around it. There's no single method that will automatically find every secret door, other than magic, maybe. But for every secret door, there's at least one method that skips the die roll.
But the real difference between hidden doors and secret doors is the mechanism for opening. My "default" secret door is the pivoting section of wall; as long as there's no locking mechanism, it opens automatically when pushed on the proper edge. Sliding panels must be pushed and slid in a specific direction. Mechanically operated secret doors have the cliché wall sconce, hidden crank, or other trigger that causes the door to open. Magic secret doors either open when a magic word is spoken or action performed, or the magic word/action unlocks the secret door, allowing it to be opened by pushing or sliding.
Mechanical secret doors can be automatically found if you find and operate the trigger. Consider for example a secret "portcullis", a section of wall that raises when an hidden crank wheel is turned. Finding the crank wheel may be easy, if you search the right location. The secret door becomes obvious if you dare to crank the wheel, opening the door.