We must be playing two different games because I've never had anyone avoid my WM unless they see it either by approaching and backing off prior to the fire (which it completely reveals itself) or with detection. Likewise I've ran banshees head first into WM a number of times. There is too much going on in this game to have some superman like vision of a small defect in the terrain unless you're playing at 10 APM just staring at your army as it walks across the battlefield.