The phrase "guilty secret" refers to something that a person knows or does that they feel ashamed or remorseful about. It is something they keep hidden or keep to themselves because it makes them feel guilty. Full definition
In the brief pages that follow, he does just that, struggling with a demon known as Nightmare while uncovering this patient's guilty secret along the way. (slate.com)
Voiceprint and DNA evidence points to Jigsaw, but the film brings on a fresh group of investigators — all with their own guilty secrets, and suspicious of each other — running around the city, tripping over spectacularly mangled corpses and literally putting the pieces together. (empireonline.com)
i do nt expect anybody else to agree with me, but he is my little guilty secret outside bet to come to us. (justarsenal.com)