Confession: I nearly always tip my players off to the presence of a secret door, even if the dice say otherwise. Secret doors that remain secret are boring -- they add no value to the narrative and provide no suspense. But I use them often. Why? Because they evoke a sense of mystery and suspense when they're found. As I've opined about traps in a recent post, I also feel that DMs who focus on the mechanical value of secret doors are missing the point. The point of a secret door as a dungeon element is as a road sign that someone wants something to remain hidden. It's a chance to pause at the threshold and wonder what's so important behind there that it merits concealment. It gets interesting when you open it.
The only time I make a secret door hard to find is when the players have been tipped off to look for one. Then you can make finding it (and possibly unlocking it) a part of the narrative. Case in point: in an adventure I'm writing, the PCs are told that in a certain room there's a secret door that will allow them to sneak into the bad guys' lair. The adventure becomes about figuring out how the door is concealed and how to open it, rather than a matter of rolling dice until it's revealed. The best kind of secret door is like the Gate of Moria, when you know where it is, but have to sweat a little before you open it, and hey, what are those ripples coming from the pond?