Well you can easily prove it with the ticket stub for the movie and the time of the ticket issued to you by the officer. And if you did get a ticket, just take pictures of you sitting in the theater afterwards. Besides, I don't think it would ever come to that, the chances of a cop catching you in a parking garage are pretty slim.
If a cop is messing with you smoking pot in a movie theater parking lot, he is already having a very bad day and the ticket is probably not what you need to worry about.