I was helping out a friend at a store today. In walks what is 100% sure MPA (older EE probably about 40), which I helped. I really wanted to find out where she came from. Added wrinkle --> she seemed to be interested in me. Now there is a massage parlour about 2 blocks away but is way to public.
What should I have done and what should I do?
You did what you should have: you served your customer, pleasantly, politely and professionally we hope.
So you're basing your suspicions of her on a MP being nearby—and imagining she had the balls to walk through its door, but you don't—and now you think strangers on TERB are gonna give you a magic pick-up line for the next time you help in your friend's store?
The first thing to do is see if Friend is OK with you hitting on his customers, but I'd say the easiest and most fun would be to realize no one much cares to spend time looking at massage parlour doors on the off chance they'll recognize someone going in or out. Without your help, your friend should be too busy. How slowly do you go through a door anyway?
Hope that she was on-shift, visit at about the same time and if she's not a MPA enjoy your consolation prize.
And for unsolicited advice: Stop imagining you—or anyone—can tell 100% a woman is a whore from appearances.