This is what down low means as per one of the sites I found.
A man, who for all intents and purposes, is straight, however once in a while likes to engage in gay sex.
I sometimes am so oblivious to what’s going on around me. Turns out we may have someone in our office building who is on the DOWN LOW. Makes me sad actually as he has a family and three children. I only know him as he also uses the smoking terrace situated on the roof of the office block. One of his coworkers know William.
I am not judging, I hope his family never find out.