How abut the possibility that she planned to go back herself and retriev it after she figured out what to do with it/about it - much like the other theories that have her interring/placing Caylee's body somewhere at first and then deciding to move her to wherever she is now???
Yes, the carjacking/kidnapping theory seems to be losing it's appeal because of the fact that Tony picked her up. Why would she use that when he could give testiment to the opposite? Your idea seems more plausible.