At about 5:30 tonight I saw a FRGU at low tide in the bay just  off the SF Bay Trail between Meeker Slough and 51st St bridge, north of Pt Isabel.  A small gull among CAGUs with a black head, thick white arcs around the eyes, red bill, dark red legs, medium grey back and large white primary tip spots.   Captured all these features in photos, albeit from behind, and then I blinked and it was gone.  Pictures at  https://ebird.org/checklist/S107991088.