The incident was caught on the video surveillance cameras of nearby businesses on Sunset Avenue.
Employees at a barber shop noticed the infant after the car drove off, and they contacted police.
People who work nearby said they were shocked.
The mother of the baby boy talked with police and is not considered a suspect, police said.
The child is just 16 days old, police said.
The infant was placed into protective custody with Solano County Child Protective Services.
The baby had been transported to a local hospital and appeared to be in good health.