Yes, he does lie down in places of traffic, but he doesn't guard it. Everyone can step over him or ask him to move and he does.
However, this growling only seems to be happening if I am in the room and lying down or sitting at my desk. My back is to the door then, and Zeus is asleep. Could it be that he thinks he owns me? I have never allowed him to be dominant. Has to wait at doors before going in or out. No toys unless I give them to him and then take them back. He brings me anything he has if I ask him to. After the growl, I asked him to go to my son and he did, tail wagging and body low.
Perhaps this is just the beginning of his guarding behavior?