We have no evidence that O-Chul ever used Detect Evil on or near the MitD.
From a meta-story point of view, if O-Chul is, as often said, meant to represent "the right way to play a paladin", part of that would be eschewing the over-use of Detect Evil as an all-purpose radar to determine how you should be acting/treating people (the way Miko uses it). A well played paladin would use it, like an honest cop, only to test/confirm suspicions that are already solidly founded.