in the closet with the light on 

When someone is homosexual but for whatever reason will not tell others and often pretend to be straight, though it is obvious to everyone around them they are not be straight due to social cues and stereotypical ways of dressing or acting around those of the same sex. Though they may be in the closet, it is obvious to everyone as the light in the closet gives away their presence within the otherwise dark walls of denial.

Observer 1:"Hey did you see the way she was flirting with all of those girls at the softball game yesterday?"
Observer 2: "Yeah I guess but doesn't she consider herself straight?"
Observer 1: "I'm not too sure about that, I'd say she is in the closet with the light on."

Example 2: Kristen Stewart.

in the closet with the light on by Melcor May 20, 2014