Years ago on my first road trip in southern Spain, I parked in Seville at a lot with a guard in the town center. Clothing but not valuables were inside the sedan trunk, not a hatchback, fully covered. I followed all the advice, including not opening the trunk after parking. The sign said pay on departure, so off I went. When I returned, the trunk had been pried open and things were missing. The guard shrugged, and walked off. I filed a police report and contacted the rental company.
Later on that night at the hotel where I had to stay while the car rental company was sorting out a replacement, a local gentleman told me what had gone wrong. The guard had expected his tip in advance to pay attention to my car, and I had no idea of this.
After that, I always parked at a lot with a guard, and I always asked his name, inquired if he was going to be there for the length of time I was planning to stay ...and I tipped him, letting him know I'd also be looking for him when I returned. Following the local knowledge, the rest of my trip was incident free. Perhaps things have changed in southern Spain, but I just wanted to share.