This guy I knew had tons of overo characteristics (wide facial stripe, white chunk on both sides of his neck, white on his legs that went nearly up to his belly), you just couldn't really see it unless he was super clean or wet.
It's the same with my girl Lacey, she has 3 high (for an Arab) whites but you would never know.
Anyhoo, I bet you'll be surprised next spring/summer when you give him a bath!
Here's a bad picture of the guy I knew, just for funnsies. I always thought of him as being your standard ranch-type QH/Paint - the kind you breed when you just want a good using horse.
ETA: in the case of this guy^^ he had fleabites everywhere he was gray (where the black skin was). Where ever pink skin was (his markings), his hair was just white.