Since several have asked, given the rarity and coincidence of these records, this Marsh Sandpiper is almost certainly the same bird as the 2016 individual. The 2016 bird foraged within a few hundred yards on either side of the current bird's location, and flew exactly over the spot where the current bird is, between Lots C and D. It was found on Apr 16, 2016.
It is also probably the same as the 2014 bird from Liberty Island Rd (about 15 miles SSE of the 2016 and 2018 birds).
As far as I can tell, here are all the records of Marsh Sandpiper in North America not including some from western Alaska.
Oct 12-16, 2011, Estero Punta Banda, Baja California, Mexico
Oct 26, 2013, Mecca, Riverside County
April 9-12, 2014, Liberty Island Rd, Solano County
April 16-23, 2016, Yolo Bypass Wildlife Area, Yolo County
April 15- , 2018, Yolo Bypass Wildlife Area , Yolo County
There's a remote possibility that ALL these records pertain to the same bird, but that might be a bit presumptuous.