Then it must be mutual "not noticing," as it looks here as though Faramir doesn't see or realize the striking resemblance to himself in Mairon's current "fair-seeming" appearance that he is shocked to learn about seven years later in "The Bridge"!

Author's Response: I'm thinking of family reunions, when two people share a very strong family resemblance and are surprised when it's pointed out. They deny they look alike, since they're only aware of the differences. Faramir only noticed Sauron's height and voice and mannerisms, all of which were different from his own.