I played a game where I proxied DTs and my opponent went in base Dark Shrine, and he happened to make TWO cannons near his minerals and they warped in right as my DTs got to his base. I watched the replay he didn't scout my proxies.
I understand he could have counted that there was one pylon missing but TWO cannons?
I have never seen people build blind cannons in the pro-scene, understanding that if it was anything else they would be incredibly behind.
Perhaps maphack? Not sure tho.