Has anyone tried calling the number to ask the people what it was about? Why not just call and ask say, "uh, yeah, hi, there has been this photo of a note left on the Grove, with your number on it, and a strange message, we were wondering if you could tell us what it means."
As far as investigators leaving notes, when I first moved to FL, I had a job installing satellite dishes. On one install the home owner was complaining how the cops burned down his field of pot, and they left a note saying they were sorry they missed him. Well, my uncle who had worked in law enforcement in that county, knew exactly what field the man was talking about, and said yeah, they did leave a note.
I guess cops have a sense of humor too.