Nick says "as if he were alert for something more" in this excerpt from the first chapter of The Great Gatsby:What is the meaning of "alert"?
“You will,” I answered shortly. “You will if you stay in the East.”
“Oh, I'll stay in the East, don't you worry,” he said, glancing at Daisy and then back at me as if he were alert for something more. “I'd be a God Damned fool to live anywhere else.”
Earlier in Chapter One, Nick describes Tom Buchanan as having a "rather hard mouth and a supercilious manner." Later, in the passage cited above, Tom's haughtiness is demonstrated as he responds to Nick's subtle insult that if he stays in the opulent East Egg he may well need to know what the bond business is. Also, Nick's remark suggests the penchant of the rich to waste money in the 1920s.
The supercilious Tom finds Nick's suggestion that he will spend too much money insulting, and he retorts that he will remain in the East. The phrase "As if he were alert for something more" implies that the "fractious" Tom Buchanan perceives Nick's remark as a challenge to his practical sense; like a fighter, he anticipates another insult. So, to reaffirm his ability to live among the very rich, Tom asserts that he would be foolish to live anywhere else. It is also noteworthy that Tom often glances around for approval, and in this particular scene, Jordan Baker provides this approval, saying suddenly, "Absolutely!" However, her exclamation is insincere and uttered only because it is what Tom has wanted since she subsequently yawns.