Airship

Heard approaching quiet propellers - too quiet for a helicopter, too low for general aviation.  This airship flew over the house, out as far as Foothill Expressway, turned around and came back.  

Was sighted landing back at Moffett Field. Highly likely to be from
 https://www.ltaresearch.com/ a Sergey Brin company. 
https://techcrunch.com/2021/02/25/sergey-brins-airship-will-use-worlds-biggest-mobile-hydrogen-fuel-cell/

Not so big as Airship Eureka - my last airship ride, in 2010.