GM: Because you're 15th level and this wall SHOULD challenge you.
PC: So you're saying that some magical force of the universe makes this same exact wall more than twice as hard as it used to be, only for the reason that I got better at climbing walls so walls got harder to climb???
You know what though, I've actually had this conversation with my players, and we've termed it "the level quandary." Which is a terrible name I'm sure, but that's what we called it. The idea being, Bing! You just leveled up, you just got better at a lot of things, and actually gained the ability to do some new things. Sweet! Guess what? The world around you just scaled to that new level of awesomeness, and things are going to be just as difficult as they were before, if not even a little bit harder. It is for this very reason that no matter what level my players' PCs are at, I always make sure to throw in, on less than rare occasions, a mob that they could literally stand around and watch attack them to no effect, then draw a weapon and slice and dice them all like so much Christmas ham; AND, an enemy or group of enemies that they HAVE to run away from, that they will either retreat or die. To me it presents a more realistic world, because dragons.
Don't get me wrong though, some things SHOULD scale.
Usually those are opposed rolls. The DC for a first level guy to perceive a sneaky goblin should be pretty low, and he can hit it with a low Perception. The DC for a 15th level guy to perceive a stealthy CR 15 monster should be much higher (because most stealthy CR 15 monsters have high stealth skill bonuses), but the guy has much higher Perception so that's fair.
A BBEG for a low-level adventure (perhaps a LLEG) might have weak traps and cruddy secret doors that are easy to find with low Perception stats while a true BBEG for a 15th level adventure might make harder traps and secreter [sic] doors that require higher Perception checks. That makes sense.
But standard stuff is always standard with a standard DC that remains the same, no matter what level you are when you try it. Or at least it should be.
amusing that the secret door is actually not able to be easily detected by dwarves with stone cunning while being a dwarf made secret door. What dread secret did the artist need to hide from his fellow dwarves? Perhaps a cache of Elvish rations and fine wine he feared his kin wouldn't understand?
Not sure what's so amusing about it, if you make a secret door, you will want to make it hidden well enough that you yourself can't tell where it is without knowing. Otherwise how do you know anyone will miss it, if you can still clearly spot it with ease?
A dwarven artisan is the first person i would expect to be making a secret door that is hidden well enough to fool a dwarf's stonecunning.
The idea being, Bing! You just leveled up, you just got better at a lot of things, and actually gained the ability to do some new things. Sweet! Guess what? The world around you just scaled to that new level of awesomeness, and things are going to be just as difficult as they were before, if not even a little bit harder.
Well, I am with you when it comes to improving challenges to match their new power level. But it should be plausible - whole tribes of level 15 to 20 goblins are difficult to believe, but upgrading to battle dragons is fine. And it doesn't hurt to throw some previously difficult (and now relatively weak) foes at them, to let them feel they really improved. Because this is one of the feelings the players play this game for.
The level 7 paladin in my group was captured lately, lost his gear und had to escape alone, in underwear und equipped only with a mundane falchion. He faced a 2 HD orc warrior - and succeeded despite his equipment disadvantage and being alone. Something which wouldn't have been possible at level 1 or 2...