As the title says:
How should you react if you notice someone taking pictures of you without your consent?
Background: It happened to me yesterday on the bus that a woman, of about my age, sat across to me. At one point her cell phone flash light turned up for a split second which caught my attention. I noticed that she had her phone on the leg, slightly angled in a way that the rear camera was pointing at me while holding it with one hand.
I didn't think much of it, phones can act up every now and then so I wondered if she was doing it intentionally but gave her the benefit of a doubt and didn't do or say anything. But then it happened again just a few more stops down the road. At this point I was sure she was doing it on purpose as it was even more obvious. She didn't even turn off the camera flash and I started to feel uncomfortable about it. I am not a particular attractive guy and never experienced anything like this before so I was quite stunned and wondered what to do about it until she got out of the bus quickly after.
Just for clarification: I never met or talked to her. I didn't even look at her until the camera flashed for the first time.