Why didn't Dwight ever suspect Creed of something nefarious?

Because Dwight can't see what's right in front of him. That's one of the ironic things that are amusing about Dwight, and his lack of common sense is preventing him from putting these particular two and two together. It's also why he keeps falling for Jim and Pam's pranks; you would expect a true paranoid to be a little less gullible.

Consider the season 2 episode The Secret, where Dwight explains to the camera about telltale signs that a person is lying. But in a later scene, when Michael (who's a horrible liar) lies to Dwight while displaying every sign on Dwight's list, Dwight surmises that he's telling the truth. Simply put, Dwight might think he's insightful, but he really isn't.

A similar technique was employed in another Greg Daniels show, King of the Hill, with the character of Dale Gribble. A paranoid conspiracy theorist, Dale suspects everything and everyone, but is somehow unaware that his wife has been cheating on him for years with Native American friend John Redcorn and that his son is really Redcorn's son - even though it's all extremely obvious and everyone else in town knows about it.

It's also worth mentioning that Dwight thinks very little of his co-workers, and actively seeks them out mainly if they 1. mess with him (like Jim and Pam always do) or 2. threaten his work status and relationship with Michael (like Andy did). And since Creed is not really one for pranks and for the most part has zero ambition at work, he's just not on Dwight's radar.