I don't believe that Casey thought that her mother was REALLY on the phone to the law when they were in the car ~
Cindy probably threatened Casey all the time with calling the police on her for one thing or another, and, she never followed through with it.
When Casey told Cindy that Caylee had been "kidnapped", and had been gone for a month, she probably figured that she could talk her mother out of calling the police, and, to go into "cohoots" with her and let her join the private investigation that she claims she had been doing for a month ~
And, Cindy is just as twisted and devious as her daughter, and, she probably thought that she COULD have managed the job.
But, with Lee showing up, it put a twist in their skirts and put them both on the spot to do something ~ IMO.
I have long maintained that had the police never been called, that Caylee would have "gone to summer with her new-found grandparents for bonding", and come fall, it would have extended, and that would have been the end of it.
Cindy would tell all that Casey was enrolled in college, was spreading her wings, was doing great and marvalous things, and, "Oh, you just missed Caylee, we had her over the weekend unexpectedly". Damn the luck.....will be sure to let you know the next time we have her, she's getting so big~!!