They've all got good senses of smell. Thankfully, growing up in the sewers meant that they've learned to block things out, but if he concentrates, Leo can smell a hell of a lot. His nose is keener with his brothers, where he can track all the places they overlap and diverge with a few quick whiffs, and it's the keenest of all for Raph. 
Leo can smell whether Raph has had anything to drink; he's learning how to calculate portions, decide if it's worth picking a fight over. He can smell Raph's anger, sharp and familiar, track his excitement over a fight, even pick up on careful traces of his fear. 
He can luxuriate in Raph's smell if he wants, and breathe in deep, heady portions like a good meal. Leo used to draw it in in careful gulps like a thief, but that was before he learned how much Raph loves being smelled, and how much his brother loved to smell him in return. He can snuggle up against Raph on the couch or sit with him in the dojo, even lean on him during a star, and smell enough to read his brother like a book. 
And he can smell when Raph's turned on, the way his scent seems to ripple and change like a stone distorting water. It's hard to put the exact nature of it into words, at least ones that humans would understand.
It's all too big and rich and deep to do it justice in a way that makes sense. It's a dominating scent, something that strikes deep at the subconscious, primitive core of him which hungers for mate, while also being a scent wrapped up in more sweet memories than Leo can count. It's a sweet little message prickling over his scales, and simultaneously a yank on his mental leash dragging him near.
Either way, his mouth starts to water and he starts to get hard in his shell. If he's picking it up from another room, he needs to stop whatever he's doing and go now, go now, some deeper animal instinct driving it along. If he's with people, he's got to get himself and Raph excused as soon as possible, preferably without embarrassing either of them. 
Leo just knows that when he smells it, his mouth starts to water, and he starts to get hard in his shell. If he's picking it up from another room, he needs to stop whatever he's doing and go now, go now, some deeper animal instinct driving it along. If he's with people, he's got to get himself and Raph excused as soon as possible, preferably without embarrassing either of them. 
It's harder to be embarrassed when they're alone. When Leo gets turned on his own smell fills the air, mixing with Raph's a heavy, glistening cloud that reminds him a bit of what opium might feel like, only instead of drifting away he's hot and wanting in the moment. 
He has to stay away, sometimes, to keep from coming too fast. Being close to his brother in this state is a dizzying prospect even before they start touching each other; it's just so much. Sometimes Leo thinks his brother's full-blast scent might be enough to kill an unenhanced human, crazy as it seems--the smell of bad things can supposedly kill you, why not good ones? 
But Leo doesn't flinch when he breathes Raph in, just kisses harder. He knows he smells insanely good, too, because Raph tells him so, and there's enough trust between them to ride each other's hormonal riptides until they crash to earth together. 
On nights when Raph isn't there, Leo still has his smell, hanging in their bed like another warm covering. He'll wrap up in whatever reeks the most, touching or fingering himself, and that's how he keeps together until his mate comes home, the way he always does.