I actually had a girl take a picture of me while I was driving specifically for that purpose when I picked her up for our first date. She told me "I just need to send my friend your picture in case you murder me." Lol
I have a close friend who when I first met I asked out and she rejected me. Now when she goes on a date and texts me something like "hope he's not a serial killer", and I text her back "oh is that a dealbreaker for you? Guess we never would have worked anyway"
Honestly be way better to take a picture of the vehicle plate, then she doesn't even have to make you aware of it, and the plate is in a database, unlike your face. On top of that, she can use a police procedure that exists for making vehicle stops: touch the side of your car with a bare hand, leaving prints on the finish for forensics.
Or, IDK, maybe she did all of the above
Edit: Now that I think about it, making you aware of it might actually be part of the plan. If you were going to murder her, you might just cut the date off early instead, saving her some time and a murder