Trickier to obtain warrants in the absence of evidence of foul play, but the longer Sebastian remains missing, the greater the likelihood IMO that foul play is indeed indicated.
I understand you wrote IMO, but is that really the case, I wonder? I’m not familiar with the adage that the length of time missing people stay missing has some special bearing on, say, actual manner of disappearance. Other circumstances beyond the mere passage of time surely must be taken into account when concluding a crime has taken place, and the passage of time before a missing person is located (if ever) has its own determining factors, like proximity to two great concealers of corpses: water and wilderness, whether death is natural or unnatural.
Certainly the likelihood of being dead positively corresponds to length of time spent missing, anyway, particularly when it comes to vulnerable children. Of course, there are other foul play possibilities besides murder, I suppose.
Again, it is vanishingly rare for a young person to be missing this long to be alive no matter if the initial disappearance was voluntary, otherwise, or somewhere in between, unless they have disappeared with another known party, usually an adult relative. Children on their own also fare worse, more quickly, than adults who facilitate their own disappearance in some fashion. I struggle to imagine how Sebastien might be alive even absent foul play, and I don’t think even Seth has conceded such a thing (alive but not a victim of something) possible. All odds are equal and equally bad.
If they don't suspect foul play and there are no POIs, why would they review vehicle gps data?
Because you can’t ring, months later, an unrung bell? Wouldn’t it be malpractice by authorities to not try to rule out/in the likely possibilities using bogstandard investigative tools? It’s easier to get a car to talk than a potentially guilty human, no? And what better way to corroborate a cooperative human’s claims—citing, say, extended travel in their own personal vehicle at the relevant times—than requesting that vehicle’s data.
What happens if Sebastien’s remains are later found in a place that can’t be connected to the most viable candidates for his disappearance (family, family friends, his own friends) and appear to have been intentionally concealed/disposed of by a third party, with a manner of death indicating homicide or undetermined, but under conditions that have destroyed potentially vital physical evidence linking a perpetrator to the crime? Someone took him there, is the obvious theory. GPS could form the link, but why not try to capture that data now rather than later? Successfully applying for a warrant to access this information is usually easier than indefinitely seizing a car, I believe, if the owner is not already in custody on a warrant that requires more rigorous cause.