I just read about a similar experience in this forum and want to share our story so others won't be scammed. In June my husband and I were in Granada heading to the Alhambra. We were almost there, curving to the left when a man stood in the road and flagged us to stop. He said the parking lot was full and we could park on the right side where 2-3 other cars were parked. He said it would be 20 Euros and pointed us across the street to a short path to the Alhambra. We took the path and upon arriving to the official parking lot, observed that it was not full at all. And of course it would have cost much less to park there. We immediately returned to our car, but the guy was gone.
The worst part is we recently received a letter from Avis Spain indicating that they were charging us a fee because of a request from the Granada government for the rental car user's name and data. We were not aware of having broken any laws and there was never a ticket on our car. I'm wondering if this also is related to the scam, and will be contacting the consulate here to report and resolve this issue.
Please don't let yourself get suckered!