I have noticed how a Mage can only teleport through walls and ceilings if the player is close enough to the actual enemy. It is indeed random WHEN the Mage decides to teleport through it to come for you, but what I don't think has been answered yet is HOW FAR exactly you need to be for the Mage to notice you.
I kinda need to know this because the boss I'm working on sometimes gets messed up when a Mage that hasn't been "spawned" yet decides to teleport through the walls when uncalled for. It would be nice to know what thickness, in tiles, is the minimum before a Mage can know to teleport to you from the other side (assuming you are always hugging that wall).
Btw, I'm using Null Space: RC2.