Dew, looking in a totally different place (obvioulsy tired!!) yes I can see it, perhaps this answers the question then. In the second picture the
'item' looks destinctively triangular where in the colour picture it just looks square? Well spotted.
oh I'm sure it's a genuine object - the higher res of the b&w shot just reveals the shape a bit better than the lower res colour shot. The object
may well be low to the ground so won't cast a large shadow, or might even be an oil slick or similar mark on the ground?