On his neck
spoiler warning!
Interesting thing... Denzel is supposed to have been in the joint for the last 5 years.. but when he meets his kid on the basket-ball court for the first time there is a definite hickey on his neck (his right side) which gets less or more visible depending on what scene he's playing. You'd think that they'd cover that over with make-up... unless they wanted to suggest that he got it from someone in the joint? :0)