I read of a case where there was a contest with Pepsi, I think it was, where you could win a million dollars if you had the winning number under the label or some such. Anyhow, a restuarant employee sold a customer a Pepsi. The customer drank it and threw the empty container in the trash and left. The employee got it out of the trash and looked under the label and won the million. The customer sued her claiming it was her purchase that won the money. The court ruled in favor of the employee.
I remember that.
The details were slightly different, but you got the gist of the story.
Rules of all competitions say no purchase necesary