People who are so obviously gay but aren't telling it. There is a difference between just "in the closet" and "in the GLASS closet".
Those just "in the closet" have speculations circulating around them gay - whether'd about their mannerism or personality, but they're not really pushing their gayness. They will tend to have opposite gender "partners", but many people will conclude that they're just beards
Those in the "glass closet" are walking and breathing of gayness - e.i., they will be always walking with that one same gendered partner, they adopt children and they might even be reported living with the partner, but they'll never say they're gay or straight.
Queen Latifah, Jodie Foster, Anderson Cooper, Kevin Spacey, Victor Garber (Mr Andrews - the Titanic guy) & Fiona Shaw (Harry Potter's Aunt Petunia) are in the glass closet.