In the begining when Bond goes to the casino, he gives the vallet man keys. Afterwards he walks to his car in the parking lots and uses the keys that he gave to the valet man.
If you have ever had your car parked by a valet, you will notice tat they like to keep the keys on a board close to the door. Bond could have easily grabbed them on the way out. Also, being a man of action, he would definately have a second set in his pocket for emergency get aways.