A few years ago I noticed a “Not Art” stencil for the first time, on a concrete barrier alongside the Mystic Path, which runs along the eastern shore of the Mystic Lakes. Over the winter I took pictures of every one I saw in Medford, Arlington and the Middlesex Fells. I didn’t go looking for them; I just documented what I encountered. And no, I didn’t know they were a thing until I Googled them. Here’s a Boston magazine story about them from 2014.