Boy Nobody (Boy Nobody, #1) by Allen Zadoff. Found in face-out fiction under Z. Boy Nobody is the perennial new kid in school, the one few notice and nobody thinks much about. He shows up in a new high school in a new town under a new name, makes a few friends, and doesn't stay long. Just long enough for someone in his new friend's family to die -- of "natural causes." Mission accomplished.
Available in Caravel Academy Library. Told from various viewpoints, Gabie and Drew set out to prove that their missing co-worker Kayla is not dead, and to find her before she is, while the police search for her body and the man who abducted her.