You should be grateful the kid was not left strapped into the baby seat!

 

Your garage has the gear to reset the light without driving the car around. Maybe the mechanic took it to verify everything was OK, but this should only have been done with your prior knowledge and consent. It's time to find another service agent.
Before going ballistic though, you should have a serious talk with your husband. He might have put the seat there because he's just dropping hints while he works up the courage to discuss adding another member to your family with you !