Sherlock, for all his attempts to seem otherwise, is actually quite human. He requires food and sleep (much as he may deny himself those necessities); he prefers his flat cold and the weather hot; he's a brilliant boxer.
And, like many people, he puts on a bit of weight during the winter months.
It isn't noticeable to anyone who sees him fully clothed. His shirts still fit the same, stretching tight across his shoulders and chest when he rests his hands on his hips and flattening along his slim stomach. He doesn't need to fasten his belt on a different loop. He doesn't even really bother to keep track of the weight, except perhaps if he's bored and thinks something interesting might come of it.
Lestrade is the only one who notices; the only one who has the privilege of noticing. He notices it at the crime scene after he's spent a good amount of time looking at the detective - more time than he probably should, admittedly - and his gaze falls just above Sherlock's belt. He sees a bit of roundness there - barely perceptible; might just be a shadow - but it sends a jolt through him all the same. He notices it when they're alone and he grasps Sherlock by the hips, his hands coming to rest on actual flesh rather than bone-thinly-concealed-by-flesh.
He notices it in bed, when his careful hands have divested Sherlock of his plain cotton tee and are exploring the taut chest. He watches as they wander over the flawless skin, slipping lower, smoothing over the not-quite-bony hips and drifting over the subtle roundness of Sherlock's lower belly. He ducks his head and runs searing lips across the understated swell of the stomach, relishing Sherlock's soft sigh and the fingers that thread through his hair, long and graceful, pressing him lower still.
And later, when they're tangled together, he notices it pressed against his lower back and it sends a shiver down his spine. Sherlock reaches around to twine their fingers together and they lay still, dragging deep breaths, overheated bodies temporarily immune to the slight chill in the air, and Lestrade may even whisper, "Beautiful," before sleep carries him off.