After posting to Tweeters last night about squid and Swallow-tailed Gull breeding behavior, I realized I missed talking about how The Gull could have ended up in the North Pacific.
On the webpage about the California Bird Records Committee vote (http://creagrus.home.montereybay.com/stgu_votes.html), Don Roberson connects the occurrence of extralimital Swallow-tailed Gulls to El Niņo events. The El Niņo of 1982 was very strong, causing many birds to disperse. Roberson suggests that perhaps a gull flew off to the northwest to the North Pacific, where it wandered for several years until was found in California in 1985. Then, he writes, "The bird behavior suggests it was responding as if on the breeding grounds."
The last El Niņo event of 2014-2016, like that of 1982, was one of the strongest on record. It's occurrence in North America also could be explained by Roberson's hypothesis. The two events are strikingly similar.