The first time Sherlock saw it was after that first chase, the first of many, the one he used to show John that his limp was indeed psychosomatic. The chase on their first night together, dashing out of Angelo's, Sherlock nearly getting hit by a car before planning the best route to follow the cab. They climbed and jumped and dashed this way and that, and in the end, it was pointless.
They'd run back to the flat, the flat that Sherlock was pleased to tell Mrs Hudson would be theirs, John's limp magically gone, both of them leaning against the wall breathless and giggling like schoolgirls.
That was when he first saw it. Joy.