Some thoughts;
There are a lot of case details that have multiple valid interpretations that we can't distinguish between - but two items I feel we know with very high certainty:
1. The killer was aware of Liz and Sergio's schedule.
2. The vehicle was a Nissan Frontier Pro 4x and the killer was driving.
So focusing on those; who would know Liz and Sergio's morning schedule?
- Neighbors
- family members of either
- possibly coworkers of either
- possibly 501st club members
- someone who spent time casing the neighborhood at least one day in advance; e.g. Thursday AM.
- someone hired by any of the aforementioned.
Who would have access to an as-yet untraceable Frontier Pro 4x?
- If this was the personal vehicle of a family member, neighbor, coworker, or 501st club member - it would be nearly inconceivable that the crime would not have been solved by now.
- If this vehicle spent time in prior days casing the neighborhood, LE would also be aware of it by now (maybe just haven't let this information out...)
So - that leaves "in-circle" people who were able to obtain a loaner vehicle whose existence other "in-circle" people aren't generally aware of, or out-of-circle people who cased the neighborhood in a different vehicle beforehand, were informed/hired by in-circle people.. or had occasion to be in the neighborhood at this time regularly enough in a different vehicle to know these things organically (Bus driver, delivery person, road/construction worker, realtor, neighbors maid/baby sitter/relative).
I'm feeling like this isn't going to be a disgruntled 501st member in their own truck - they'd have been identified by now. Same with uncle xyz or an angry coworker. This is either a stranger, or a non-stranger in essentially a strangers truck... And the killer didn't just stumble onto garage sale signs Thursday night and take advantage of an opportunity - such a perp wouldn't have known Sergio's schedule.
This person was also away from their normal activities for at least 7 hours Thursday night into Friday morning... More if they had to obtain and return a truck. If that trucks owner wasn't aware of it's use, how far could the perpetrator have driven it and still gotten the truck back in time for it not to be missed? I'd think a 7am murder would be pushing it.
Also I think commercial and rental vehicles would often have some sign/symbol/insignia that could be seen in footage (although some of those are magnetic).. so my inclination is that this is a private vehicle... most likely owned by the killer or accomplice or it would have been dumped and found somewhere or reported missing.
Making some logical leaps, it sounds like someone from outside of the area, in their own vehicle, having received logistical information from someone close to Liz.
All of course conjecture and MOO ...