Just realized that Z had used the symbol twice, on the Halloween card itself as well as on the envelope. So in fact it was placed there by purpose rather than actually having drawn it out of a temporary mood. Obviously Z wanted to give a hint or task to figure out what it may represent.

QT

He placed it where you would expect the return address to be.
Maybe the idea that you should overlay it on a map isn’t so far fetched: viewtopic.php?f=63&t=5164

It’s not far fetched at all. As I mentioned in the other thread, you could even rotate the symbol so that the "Z" becomes "N" for north. The issue is all those right angles are going to line up streets and roads out the wahzoo. Better luck validating the Z13.