Da Vinci did this. My mom got me a book when she went to Italy, to Florence...She asked me what I might want as a souvenier. I said, "Anything having to do with Da Vinci." The book is a biography of Da Vinci, written in Italy by an Italian. Whether that makes it better or more accurate I don't know. In it, there was a passage about Da Vinci and his mirror writing. Most of his notes were done that way. He didn't write in the mirror - that's the amazing part - the notes could only be read in a mirror. The book said he did this because he thought evil spirits looked over his shoulder while he wrote in order to steal his ideas. This mirror writing was a way to throw them off.
Sounds to me like the tinfoil hat people who think aliens are stealing the thoughts out of their heads. Or evil telepaths...whatever.
I tried some mirror writing today. Writing in a way that could only be read in a mirror. It ain't great, needs practice, obviously, but I got the gist of it, I think.

Peace,
Barry
PS: Give it a try. It's fun.