Yup, that appears to have been what it was....swept that area and the "gremlin" vanished. Thanks for the idea, bigredfish and bp2008....somewhat surprised that the silk strand didn't show up at night, but it would've been very small. Mystery solved!
Just wondered if anyone else has ever seen this sort of "nit" moving around in the image? It appears at the bottom of the central post at the start. It's inside the camera I reckon owing to my having cleaned the external window but seeing no evidence of anything on it anyway. At night, it's...