This adult gull looked like a female Caspian on head/body shape and bill structure. The bill and legs were pretty bright but then breeding plumage Caspian's are not exactly dull. Eye appeared mid honey. The thing that really baffled me though was the primary pattern. The underside of P10 as seen from the opposite wing seemed to show no black outer web, with the whole visible area appearing white. P9 also seemed to have an extensive white tip broken only by a narrow black line. The amount of black feeding back towards the primary coverts on the closed wing also appeared less than normal. Wing flapping briefly revealed a striking black/white striped wing-tip like a Caspian but with P9-10 appearing more like a Thayer's Gull. There was definitely black on the outer edge of P10 but it was minimal. I gave a lot of thought to it possibly being a yellow legged argentatus but the birds shape and jizz just seemed too Caspian like. I was quite relieved when it flew off.
