I would always notice anything on my windscreen before I start the engine. So if somebody left something there I would unlock the door, sit down, notice it, get back out and remove it, and then sit in and start the car. They can't get me like that.
Perhaps I would be vulnerable when I got out to remove it even if the engine isn't on. They might rush me and take the key from me. Fine. But I really doubt they'd do that. that's a straight up car jacking. And if they wanted to do that, no need to put anything on the windscreen. Just rush me before I even open the door in the first place. Or hold me at gunpoint once I've started the car.
But clearly they're too scared to do either of those things. They rely on stealth. They can't stealth me because like any normal person, I would notice something on my windscreen and go to remove it before I start the car