It had been nothing more than a glance. A quick shift of his eyes from the jeans to Sherlock's torso. John probably thought he hadn't even seen it. More likely, John hadn't even realized he'd done it.

Nonetheless, it led the consulting detective to several important inferences:

1) John liked his bum. Of course, he'd already known that, so on to inference number-

2) John desired to see said bum in a pair of tight jeans. Sherlock imagined he would derive great pleasure from said jeans which would most likely lead to-

3) John ultimately removing said jeans to better view said bum.

When John and Sherlock left the shop half an hour later, Sherlock was the proud and very expectant owner of a new pair of Levi's.