[cobirds] Question about gull

Opinions are being requested about a gull I saw in January in the San Luis Valley. This gull showed up in a farmyard, about 4 miles south of the Monte Vista Wildlife Refuge. I watched it  closely for several minutes with and without bino until I finally left, as it strolled around, pecking at whatever it might.  Ground had patches of snow and clear, dry areas. All  physical features – size, bill, eye, feet, tail -  were like a Bonaparte's Gull EXCEPT that it had no dark patch on cheek. No one else observed this bird, not even the resident birders on the farm who were away. It had never been reported in winter in the San Luis Valley in the past, not even by the 2 Bobs or Rawinski. Any good suggestions??? Virginia Simmons, Del Norte