In RTS the computer always knows exactly where you are, even when there are upwards of 5 possible places for you to be. Not only that, in a free-for-all match with you and, say, 7 AI opponents they will more than likely attack you with a higher priority than the rest of them.
Dawn of War is a great game but it does this all the time. Imperial Guard are the worst for it because even when your Eldar base is fully stealthed - BAM: they land a scan right on top of you, added to the fact that they've probably already parked their entire army on top of your completely invisible base.