CHICAGO – When a police detective said last week that a man suspected of strangling a suburban Chicago teen in 1976 may have killed as many as a dozen girls and young women, the question that screamed louder than all others was: How did nobody notice? Today, as Lisle Police Detectives Chris Loudon and detectives in other communities where Bruce Lindahl lived try to retrace his steps, what is emerging is a terrifying murder mystery created by a man Loudon describes as a serial killer, a monster hiding in plain sight. “Bruce stayed under the radar,” he said. When authorities discovered Lindahl’s body, dead from a knife wound, in a Naperville apartment in 1981, he was known to various police departments as a loser who had been arrested many times but who had no felony convictions on his record.