My partner usually picks him up in the evening. About 6 months ago she was busy so I did it. I picked him up, drove home, parked the car in the garage and walked straight into the house. Something felt wrong, but it was maybe 30 seconds until I realised I'd left him in the car. He was completely fine, but it put the fear of God into me.
Anyway, I'll contribute my poorly thought out technical solution - a thermal camera that monitors the seats for warm bodies, if you lock the car when present it beeps the horn.