IMOO It's possible that, if 2 teenage boys live in the house where the bike was found, that maybe they found the bike abandoned and just took it home. Maybe they are innocent of anything to do with Autumn's death. Just picked up an abandoned bike??? Maybe they called, or their mom, LE when they realized whose bike it was. Maybe, Maybe???
This is probably one reason why police aren't naming any suspects yet or making arrests - in spite of how bad it looks for these boys to have the bike found so close to where Autumn's body was found. LE has to have solid evidence linking them to her body, motive, etc., otherwise a good lawyer could use the very scenario you came up with as a defense to win an acquittal.