I field tested this yesterday and it worked brilliantly.

I was out with a good friend. While I sat smoking outside the bar I received a text message from a girl("Oh you are here too "), but I didn't know who she was. She had my number but I didn't have hers.

So I thought by myself. I'm going to open the hottest girl around here and my opener went something like this.

Me."Hey, I'm trying to find out who my secret admirer is coz i've been getting these text messages from an unknown number ..."

(show her quickly)

HB. Haha it's not me.

Me. "You sure? Coz i've spotted you eyefucking me the whole evening" (small grin)

HB. "Hahaha" (At least her BL was quite good)

Me. "You don't have to be shy about it,actually that's very brave. It's nice meeting you. I'm Name, what's yours?."

HB. Blablabla

Then some smalltalk and rapport building.

Btw. I never tried to find out who really texted me.