I noted before something along your lines. Let's suppose that Sergio WAS home, OK? He would have heard the shots and ran outside, which would have been normal-or, he would have not responded, which would have looked highly suspicious. Had he run outside, he would have seen the truck as it returned to the crime and possibly noted the plate number or partial. Either way, it is a big issue. In my opinion, ONLY if the killer knew that Sergio was gone could he/she have known that returning to the scene would be non-confrontational. Now, there was 2 1/2 minutes between the murder and when the perp returned, so any of a number of residents could have seen the truck return as well, but we know that at the VERY same time that Sergio left for work the perp left the Goddard School parking lot to conceal on a different street.
Food for thought...