Sometimes you do it for a moment of suspense, and it works just fine, but most of the time, the PC's delve at a leasurely pace.
Perhaps, but remember, to Take 20 on Searching every 5' square requires 4 hours for
one 30x30x10 room.
So either they aren't going to search
every single square that way... or you have a more extreme definition of "leisurely" than I'm used to
Now think like a crazed and cunning architect of a secret-passage-ridden dungeon. Figure out where the most obvious, sensible, and traditional places to put secret doors are.
Behind the altar/throne/statue. In the closet of the high priest's bedchamber. In the floor beneath the carpet in the great hall.
Don't put any secret doors there.
Figure out where the very, very least obvious places are.
Don't put any secret doors there either.
Now find somewhere between the two.
That's where your secret doors go.
Since there's nothing unusual about the location of the door - the people who use it just know it's there - the characters will only be Taking 20 to Search there in one of three situations:
1. They have reason to believe it's there - the evil necromancer disappeared somewhere in this unexceptional hallway.
2. They're Searching
everything. Which can easily take
days.
3. They have a hunch. Not much you can do about that
And in any of those three situations, I don't mind at all that a person with a good Search score is guaranteed to find it by Taking 20.
But if they want to take things at a reasonable pace - one or two rounds per square, tops - they need to max out that Search skill, and hope for decent rolls.
-Hyp.