Secret Hot

A person that you don't realize is attractive either because of the way they dress (like a work uniform) or because of your relationship to them (a boss or blood-relative).

In Death to Smoochy Catherine Keener sees Edward Norton take off his costume and she realizes he is secret hot.

Guy #1: Did you see Jessica come in to pick up her paycheck?
Guy#2: She's totally secret hot, these Toys-R-Us shirts can really hide a great pair of chesticles.