spray painted hearts

For a little less than a year now I've been noticing these hearts all around Miami. I see them everywhere...spray painted in various parts of the city. Kendall, South Miami, the Gables...everywhere! 

I'm convinced that there's some hopeless romantic delinquent running around Miami at the wee hours of the morning spray painting hearts to show their affections for their one and only. Picture it...it's great. 

Has anyone else seen these? Take a look around, they're everywhere. 
Let me know any interesting theories you might have on where these things come from...