The old stones in Smithfield

This is an alleyway in Smithfield Meat Market London. One cold morning I had been walking around the Barbican and decided to head west in search of a cafe for lunch. As I walked past this alley an icy gust stopped me in my tracks, I always carry a camera in my bag and as I crouched down to rummage around for it I saw a movement in the corner of my eye. Something had moved across the alley from left to right, I grabbed the camera and quickly took a photo, this one above. I walked up down for ages examining the walls and the stones in search of a clue I even knocked on the door that you just see on the right but there was no answer and I found no clues. This photo was taken in 2013 and each time I see it I think about that experience and wonder what it was.