Question: Isn't an old school D&D "detect secret doors" roll the equivalent of a "skill check" today? Wouldn't that "detect secret doors" be the same thing as rolling to see if a character found a scrap of paper in the fireplace or a bloodstain on the carpet? I'm not seeing the distinction.
Answer: Here's the distinction. If an old school D&D player told the DM he was looking in the fireplace or examining the carpet then an old school DM wouldn't ask for a roll. If the player describes what his character is doing, then it's just common sense that his eyes would be working just fine and he would discover the scrap of paper or carpet bloodstain.
The detect secret doors is for automatically noticing if there's a secret door just by walking near it. Again, if a player was specifically checking for secret doors in a particular room or hallway, then there's no roll. Given enough time (they can't be glancing for secret doors in between rounds of combat or something), that player will always find what he's looking for unless there's some greater force at work... like magic.