There was one prototype in California at the time.
"The sole prototype reconnaissance platform, the YRB-49A, first flew on 4 May 1950. After only 13 flights, testing ended abruptly on 26 April 1951. It was then flown back to Northrop's headquarters from Edwards Air Force Base (formally Muroc) on what would be its last flight. There, this remaining flying wing sat abandoned at the edge of Northrop's Ontario airport for more than two years. It was finally ordered scrapped on 1 December 1953.
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That is a close fit to the timeline although testing reportedly stopped in April 51, although it wasn't ordered scrapped until the same month as the sighting. Doesn't mean the demo work started immediately. The flight characteristics don't fit, hovering and then hitting high speed. You would also think that Johnson and the others would at least be aware of it, although Johnson describes a saucer, one of the crew describes the crescent shape.