[Wissa]

Approximately half of those were either inbound or outbound of the US or flying parallel to the coastline north/south.

In a flight plan for landing at or taking off from coastal CA cities, they may be below the altitude where contrails would form on that day.

Planes flying parallel to the coast may be east of where the appropriate conditions existed.

I don't know in this case. I'm just throwing out possible explanations for the absence of visible contrails in the images.

[The Magical Mischief Tour]

I know, I’m a Commercial Pilot, (ATP License) with over 20 years experience flying many types corporate jets.