I've had suspicions along these lines since I found out about Nicole's murder. That June 9 date....
Think about it: This could be why LE is being so ambiguous about the forensic results. I do believe there was a dead body in the car. That much is clear.
What if, what if...Casey was involved in the murder or witnessed the murder? She could have voluntarily or been forced to transport the body, or items containing forensic evidence like the murder weapon.
Others involved could have snatched Caylee to keep Casey quiet, or Casey sent Caylee away to keep her safe because of threats.
Here is why I think LE is being deceptive in their public pronouncements: (and keep in mind all of this is just my opinion and pure speculation so I could certainly be wrong)
LE knows the body in the car is not Caylee and they know Casey must have some involvement in this other murder, but Casey won't talk. So they do everything in their power to have her labeled by the whole world as a child killer in order to pressure her into talking.
Casey and LE are now in a catch-22.
LE not only wants to know "what is your involvement with the dead body that was in your trunk?" they want to know "where is Caylee?"
If Casey brings Caylee forward, not only is the baby's life potentially in danger, but now she will have to explain what really happened.
So LE is holding the "child killer" label over Casey until she talks.
And Casey is holding Caylee back in order to avoid answering questions about the other body. As long as Caylee is missing, and LE and Casey are both using her as leverage, the poor child is in limbo. So long as LE takes this tack, she can sit back and wait it out, even WITH all of that pressure.
Casey doen't think far enough ahead to realize this can't go on forever. Right now she's out of jail, showering and doing her nails, surfing the internet. Probably as far as she's concerned some kind of deux ex machina will strike and everything will resolve itself. But eventually the truth will come out, one way or another.