We know about two cameras. Don't know how many there are around the neighborhood -- hope more, and that LE has the captures from others as well, but... .
Doubtful those types of cameras caught ALL of every kind of traffic, IMO.
Also -- some say it's a pretty busy neighborhood. Maybe something's there, on camera, that isn't standing out as unusual, but actually is pertinent.
Or -- maybe a vehicle goes down Babbling Brook and leaves shortly thereafter, not a "known to the neighborhood" vehicle. Likely those types of cameras don't get a good shot, if any, of license plates... and maybe LE hasn't been able to locate that vehicle. Maybe Tyler got in that vehicle with someone and rode away, and maybe they took Alanna with them. Maybe that vehicle later dropped Tyler off somewhere nearby but not on Babbling Brook, and he walked home. Maybe the driver promised he/she would "take care of things".
All just theory.