When I woke my iMac up this morning I noticed there were two docks. One was the normal one, and there was another beside it, with one app only in it - Safari. Mousing over it I saw a tooltip with the text "from Mac" or something like that. It was definitely on a separate dock area - there was a gap between it and the regular dock. I clicked on the Safari icon in it and Safari launched normally, and the separate dock disappeared. There is another Mac on the network - a retina MBP, if that's relevant (though I can't see how). I've never seen this before, and I wondered what it was I came across.