Infant car seats face the rear, so you wouldn't necessarily see a sleeping baby when looking back.
This is why we need alarms in the infant car seats. My car lets me know when I'm not wearing my seatbelt, or when I'm driving too close to the yellow line, or when the tire pressure is too low. Lives would be saved if car seats had alarms.
Or if parents were diligent and thoughtful enough to check the carseat every time they exit the vehicle...
Honestly, I don't think alarms will save many babies. Only parents doing their job, or putting their own reminders in place in case they are too busy or forgetful to do their jobs.
(or grandparents, aunts or uncles, caregivers, etc.)