They probably figured nobody would ever check for it. Some green thumb created a small marijuana grow site above a public park in Rowland Heights, CA, and believe it or not, someone discovered it. The Los Angeles County Sheriff's Department's Narcotics Bureau was informed that weed may be growing above Pathfinder Regional Community Park, and they indeed found "a small encampment and marijuana plants in various stages of growth."
The grow site was called "unusual" because it was located in a site not accessible to the public as other illegal grow sites that were discovered. They almost got away with it, but almost doesn't count.