It might also be that McNulty is one of those guys who does his job very well, but does not go talking much about it, or poking his nose outside his own responsibilities. Because his job is done well, no one comes poking around much, because God knows, there are enough problems elsewhere to deal with. Meanwhile he is a private kind of guy who doesn't garner or demand a lot of attention. Therefore few really know just how talented the guy really is, until his knowledge is brought to the forefront, as in with the request to interview for the St. Louis job. It may only be then that someone like Whiz discovers just how talented the guy really is.
Such a fine point.