To my assumption, Bruce might've paid Alfred's check and that's how he noticed him. Alfred drank a cordial which comes after having supper. Then, you see him putting away his credit card and looking forward in disbelief after Bruce told his waiter to tell him he didn't have to pay anything. That's what I think really happened here, one last gesture of courtesy before the end credits roll followed by a nod that gratifies everything done through the years (and Alfred's meal of course lol.