JennaLee, I think you are right. I think the reason is because they are hoping he revisits the dump site, and they want him to use the vehicle that they know is his so they can follow it (they should have a bug transmitter on it; that's been done before).
BTW I am brand-new to WS. Have read most of the five Susan P. threads, skimmed some earlier ones to catch up the last couple nights. There is an old saying that I think is very applicable to JP: "If it looks like a duck and quacks like a duck, it's a duck."
In the last couple of years I have watched every true-crime murder mystery episode I find on the cable channels, from "American Justice" hosted by Bill Kurtis (my fav), to "48 hours Mystery", "Forensic Files", "Cold Case Files", etc. There are elements from many of these past murder investigations that make JP look so guilty it's not even funny. As someone has already said here, this is a text-book case of "when hubby snapped."
But the biggest issue I think the police have to deal with is THEY HAVE NO BODY! So they can't even say it's a murder investigation! All they can do is say they "suspect foul play." I recall two cases that bear POSSIBLE similarities. One was the murder of a prominent attorney by her husband (from a city in the south USA). They never could find her body! Finally, years later, they finally convicted the husband, and here is how: The husband's father confessed to driving south to meet his son, taking the body of the wife, and he stashed it in a branch pile of a logging operation, where it was shortly burned along with the branch pile. Pray that's not what happened here. They have to be able to present evidence to a jury that convinces them "beyond a reasonable doubt", as everyone knows. And sometimes it's just better to wait for the better evidence than to jump the gun prematurely on flimsy circumstantial evidence (we all know what defense attorneys will do to explain everything away.)
In another case in the northeast USA, the husband confessed to disposing of the dismembered torso of his wife by pushing it in a sled through the snow (it actually got away from him and he had to run after it). But he simply pushed the sled into some undergrowth. A week or so later he went back and dug a shallow grave so the remains wouldn't be found after the snow melted. Other wife-murderers have also been found out because they went back to check on the burial site to make sure it hadn't been disturbed, or to look again at how well it blended in, etc. Suffice to say that JP needs to be closely monitored.
The lack of progress is frustrating to perfect strangers of SP, so all of her family, friends and even acquaintances have my most heartfelt sympathy. We could be in for a long wait for justice.
