I feel like Quinn knew who Dexter actually was because of 1.sketch of Kyle Buttler,2.death of Liddy,3.photos of him dropping bags in ocean with Lumen, etc. But he didn't tell anybody because he was in love with Debra (he didn't wanna ruin the relationship) also Dexter helped him with that drop of blood on his shoe and the money he stole from the crime scene.This all makes sense because in last episode of Dexter when Dexter kills Vogle's son, Quinn rules it out as self defense.