[Rune Son of Bjarne]

For some reason I always imagined that Saruman used his powers to conceal this from Gandalf, who would not be aware of this deceit when arriving in good faith. With Gandalf locked in the tower (which btw seems like a pretty standard place for accommodation of prisoners), there would be no need make an effort to conceal the pits and whatnot.

I have absolutely no reason to assume this, it just seemed the most plausible to me.

However, I do assume that Isengard and the surrounding area are big enough for Saruman to strategically place these installments where the cannot easily be seen.