On a hot day, should you smash the windows of parked cars with "Baby on Board!" signs, to rescue the baby?

"supposedly"? A Baby on board sign doesn't mean there is a baby in the car at that moment! It's not like an occupied sign on a bathroom or something. Look in the window and see if there is a baby! If there is a baby, I would call 911 and report it immediately. If the baby is alert and/or responds if you tap on the window, I would wait for law enforcement or for the parents, whoever gets there first. If baby is unresponsive, looks lethargic, or is upset and crying, yes, I would break the windows to rescue the child.