This is a great find!  It is hard to know if the recent records indicate some recovery or better coverage by knowledgeable observers. Many records are are concentrated in the area of Oregon House, which result largely from your interest.  But the detection of martins in the breeding season at new nearby sites (this one, Englebright Reservoir) suggests some expansion from this area which is highly welcome.  I do not see other recent breeding season records in eBird away from the few known locations in the foothills.  (Note that second-year birds are still moving through in early May).

One thing is pretty clear to me - these martins are not likely to be coming from the Central Valley. Other than in Sacramento, there have been very few breeding martins in the Valley since the 1980s (https://www.researchgate.net/publication/239616154_PURPLE_MARTIN_POPULATION_STATUS_NESTING_HABITAT_CHARACTERISTICS_AND_MANAGEMENT_IN_SACRAMENTO_CALIFORNIA.  The Sacramento colonies appear to have declined substantially again this year, but we won't know for certain until early July.  Numbers of second-year birds that begin nesting late have not been fully determined yet. 

I plan to check a record with photos of 3 birds made by knowledgeable observers in Galt on 21 May, which is very surprising. I encourage everyone to be on the lookout for martins and to report them in eBird with any evidence of breeding behavior.