I think he timed himself and knew approximately how long it would take police to respond if someone (anyone) called 911 during the crimes. With the planning he had to know there was that possibility.
I also think he had a plan to stash all of the evidence we are still wondering about - clothing, shoes, mask, weapon, etc. He probably practiced a lot and timed himself. He could have removed everything before getting in his car and had something to stash the evidence in. Maybe left his keys and phone in the car and a "stash" bag under the car while he committed the murders. I think he was almost completely covered up (not the eyebrows apparently), and had to strip out of everything (layers) before entering his vehicle. If that evidence is ever found it may be all together. Could have dropped it that night in a pre-planned location and never went near it again. He either left the sheath to toy with LE (wrongly thinking there was no chance of DNA, or any other connection back to him), or made a crucial mistake.
Interested to know what was found in his apartment. Wonder if he chatted with anyone about the crime?
IMO, he had a lot of interactions with police for someone with his background trying to plan the perfect crime.