In The Great Gatsby, what startling discovery does Tom make shortly after lunch in Chapter VII?
The answer to your question is found in this passage:
"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.
Tom, caught up in his own life and his affair with Myrtle Wilson, had paid little attention to his wife. Daisy belonged to him, just as his home, cars, and horses belonged to him. The idea that she could or would slip from his control and have a relationship with another man was not imaginable. When he realizes that Daisy has a life separate from his own, he suddenly regards her in a different light:
His mouth opened a little and he looked at Gatsby and then back at Daisy as if he had just recognized her as someone he knew a long time ago.
Tom sees Daisy as the woman she had been before he confidently had tucked her away in his mind as "my wife."