Say I believe ol' Georgie Porgie...
Casey hides Caylee's body until some point after the 24th, using the borrowed shovel, after she removes Caylee's body from the car backed up in the garage.
After the 24th, Casey figures it would be best to move Caylee's body. At this point, the body has been decomposing, thus the decomp smell and substance in the car. Casey dumps Caylee somewhere, then dumps the car. I don't know why the tow people or impound people didn't note/report the decomp smell, but Geo or Cindy notices it when they retrieve the car (what day was that again?).