Standard Detection (with a twist on dwarves)
Most systems state that you have a 1 in 6 chance of detecting secret doors. Unless you are an elf or a half elf, then you get have a 2 in 6 chance. Some even give them a 2 in 6 chance for being near it. A passive check. For secret doors, there is always a need to for an active search to find one. Concealed doors can be found without an active search.
Dwarves are not given an advantage for detecting secret doors, I always thought they should. But there is something in their natural abilities that I believe applies, the ability to detect sliding or shifting walls (I include floors and ceilings). I apply it to detecting secret doors. Dwarves have a 4 in 6 chance now. Unless there is something other than construction keeping the door secret (such a magic) then dwarves get the standard 1 in 6 chance.
The biggest difference for me between a secret door and a concealed door is detecting a secret door requires and active search while a concealed door can be found by just being within the room. A passive check of a concealed door is the same as it would be for detecting a secret door (1 in 6 chance or 2 in six for the pointed ear folk). If an active search is made then I increase the chance to 2 in 6 or 3 in 6 for the elvenkind. And the dwarves, should the situation apply, increase to 5 in 6.
Difficulty of Detecting Secret Doors
Each secret door has a simple difficulty rating, because not all secret doors were created equal. I use a simple three tier system. The bade die is a d6 in most cases, but if the creator of the door is master at what he does or magic is involved in the concealment, the die rolled could be a d8 or as high as a d10. The number the players have to roll under remain the same. So an elf detecting a very difficult secret door would have a 2 in 10 chance of detecting it.
This little addition I found on pg. 97 of the 1st edition DMG. I still find stuff in that book.
|Secret door builders tip, keep the hinges on the inside.|
Most of the time if the players find the door they can open it, but every once in a while there is a trick to getting a door open. Maybe they need a key or there is a fail safe to opening it. It's fun to watch the players reaction when they put a key in the door and you ask something as simple as "Which way do you turn the key?" and watch them pee themselves a little. In games I don't see enough trapped secret doors. I would think if you went to all that trouble to hide something you might add a little extra protection.
The other approach I have is I'll describe the door and immediate area in a little detail and its up to the player to ask questions to get more details. Then they will have to decide what to do. Maybe the push the stone that is sticking out slightly farther than the others. To push that stone locks up the secret door, now only magical intervention can gain the party access. It was the stone to the left that would have released the door. Simple things like that help keep the players engaged in what can be a boring task.