Here is an article by an ex-pat retired American police officer currently living in Mexico. He describes a common scam that is running rampant in the Cancun area. Versions of this scam happen everywhere, often to foreign tourists. It might be one of the most common scams seen worldwide.
Riviera Maya: Beware of This Scam
How do you stop it? I like his advice: “I immediately become suspicious of unsolicited help from strangers. That suspicion level grows exponentially if the person is trying to get me to accompany them to more isolated location, like a bathroom or a parking lot.”