Day after day, night after night, Francisco Hernandez Jr. rode the subway. He had a MetroCard, $10 in his pocket and a book bag on his lap. As the human tide flowed and ebbed around him, he sat impassively, a gangly 13-year-old boy in glasses and a red hoodie, speaking to no one.

After getting in trouble in class in Bensonhurst, Brooklyn, and fearing another scolding at home, he had sought refuge in the subway system. He removed the battery from his cellphone. “I didn’t want anyone to scream at me,” he said.

All told, Francisco disappeared for 11 days last month  a stretch he spent entirely in subway stations and on trains, he says, hurtling through four boroughs. And somehow he went undetected, despite a round-the-clock search by his panicked parents, relatives and family friends, the police and the Mexican Consulate.

Since Oct. 26, when a transit police officer found him in a Coney Island subway station, no one has been able to fully explain how a boy could vanish for so long in a busy train system dotted with surveillance cameras and fliers bearing his photograph.