[ARCHIVE ANSWER]
I would often smell marijuana without telling the suspects. I wanted to carry them through the process of asking for consent. This is partly how I learned body language when a person is lying. I knew they had pot (or at least had it recently), so I wanted to see their response when asked to search.
I never had any person tell me they were free to leave because the citation had been issued. If the suspects pulled that card, I would have immediately drawn my "probable cause" card and let them in on my secret. I smell pot! I would not have allowed them to leave.
This does not mean using the "free to leave" doesn't work. It just doesn't work when the odor of pot is present because the officer really doesn't need consent because he has probable cause!