How would you approach someone who took a picture of you in public?

I was on my way to work this morning and took the city train as usual. The train was fairly empty so I sat by myself and started reading a book.

A woman entered the train at the next stop and sat across from me. She got her phone out and seemed to do stuff on it, but I looked up when I heard the shutter sound to see it was pointed directly at me.

She tried to play it off as she lowered her phone, but it was very obvious what she had done. I assume she didn’t realize that her phone wasn’t on silent and, thankfully, told on her.

I didn’t say anything, but now I wish I had. To be fair, I can’t say for 100% certainty that she took a photo of me, but it seemed fairly obvious. It’s something that cannot be proven easily unless you appear on the internet.

So my question is, how would you approach someone who took a picture of you in public?