What startling discovery does tom make shortly after luncheon? How does he happen to make the discovery?

Tom is so caught up in himself and his affairs with women, like Myrtle, that he fails to see the attraction between Daisy and Tom,

"Who wants to go to town?" demanded Daisy insistently. Gatsby's eyes floated toward her. "Ah," she cried, "you look so cool."

Their eyes met, and they stared together at each other, alone in space. With an effort she glanced down at the table.

"You always look so cool," she repeated.

She had told him that she loved him, and Tom Buchanan saw. He was astounded.