> What is this mystical drill proof material behind the stucco?
Good porcelain tile (or thick glass) will defeat masonry drill bits, tile drill bits, and cobalt drill bits. I keep a set of diamond drill bits around for drilling real porcelain tile when I want to install handle rails or shower grabs in fancy bathrooms and hot tubs. If someone happened to plate the side of the house with field stone or cut stone and you are hitting a piece of granite you will not make much headway without a stone cutting bit.
I would take a cheap center punch and see what happens to the tip after a few wacks and if it looks like rock dust or something.