I saw the very same thing and I watched them like a hawk until they noticed I was watching and went rushing round the corner. I noticed they had torn my rubbish bag with general waste. I think there are looking for metal and copper as this is where the money is, hence the trolly's to carry the load. I haven't seen them since and am always checking to see if my bin bags are torn. If I did see them in my front garden again, I won't be pleasant that's for sure.