John is walking along Gower Street when he notices Lestrade coming out of a shop on the other side. John almost doesn't realize it's him - he is laughing, and holding a child on his shoulders. John slows down to watch. Two more children tag along by his side. He is unmistakable, the silver hair always giving him away. He doesn't see John, but even from this distance John notices the lines on his face don't run as deep. John smiles, and keeps walking.
Much later, John is standing around Scotland Yard, waiting for Sherlock when he sees Lestrade at his desk. He's staring at his computer intently, fingers cupping his chin. John sees a small, plush football on the desk, though, and he's bored so he approaches Lestrade. "You play?", he asks, gesturing to it. Lestrade looks up. "A little," he says. "I'm not very good." Modesty is his strong suit, so John doesn't doubt his ability.
Then, at one point Lestrade invites him over to his house for a small party. Sherlock doesn't understand the point of it, but John drags him along anyway. A few other people John recognizes from Scotland Yard are there. They're chatting in the hall, Sherlock sprawled on the sofa, when two children come running down the stairs, giggling. Lestrade immediately goes to the two girls, hushing them, and sends them back up after a few whispers. John smiles. Then, as if he hadn't seen Lestrade's children before, he asks: "You have children?" With a laugh, the other replies, "Yeah. Five, can you believe it?"
At the end of the day, John concludes that the Detective Inspector is still an enigma.