On the green hornet car,it could possibly be some type of button for a solenoid, you even have pads you can put under the paint to sense touch.
I,ve had several ideas over the years for a flush handle. Was going to make the type I think jaguar invented that you push in on 1 side & grab, which Kindig has their version.
Also thought about one on the top of the door that simply slides down when you push on it . This would be reasonably simple also, could use a piece of 1" thick UHMW plastic in a rectangular surround with springs & stops.
I did make my own interior door handles from scratch. They are in the aluminum strip in my door. You pull it out slightly & you can reach the pw & pdl swiches, but you don't see them . pretty cool huh.
Whatever I make , I,m trying to make it simple enough that others can follow & post it, like I,m doin with my steel ground effects, which I can't wait to see painted black.