This happens around the middle of Chapter 10 of the book. Argus has just dropped Percy and Grover and Annabeth off at the Greyhound station on the Upper East Side of Manhattan in New York City. Percy has just seen a "Have You Seen This Boy" flyer with his picture on it.
At that point, Grover tells Percy that Gabe has been protecting him. He tells Percy that Gabe's odor, which is "repulsively human" has been making it impossible for the monsters to detect Percy. This happens on page 159 of my copy of the book.
when grover tells him. when they were walking past the house
When Grover was walking with him, he told Percy.