In the first episode of BBC's Sherlock, "A Study In Pink" Sherlock insinuates a sexual encounter occurred between Sergeant Donovan and Anderson. He mentions something about Anderson's deodorant, and it vaporizing. Also the clear observations about Donovan's knees.
But how does he know Donovan had not been home when he first saw her? What was the connection he made about the deodorant? I really love Sherlock and the stories woven in it, but sometimes I don't understand many of Sherlock's more subtle deductions.