I was walking along an underground passage at Bank tube station and a set of businessmen were walking towards me. One of them, close to the edge near the wall, seemed to be accompanied by a small grey mouse keeping pace with him.
As he got closer, I saw that it was a big ball of fluff and hair, rolling along parallel to his left foot, presumably being carried along by his back-draught.
It kept pace with him, tumbling cheerfully and obediently, until he walked past me.
'How odd', I thought, until I saw another businessman being followed by a large, flat dustball, hovering about two feet behind him and about three inches above the ground.
Now that one was positively sinister.