You don't mention if you work in a restaurant or what? Did you bring her check to her promptly? Do they usually pay at the counter, or do you usually take the money and bring back the receipt? Did she just get tired of waiting for you, left what she thought the cost would be and got up?

There was an elderly lady, when she ordered , she was fine. Until I heard from my boss saying she on purposely left $15 on the table but the bill was $20. My boss confronted her and she paid the right amount. She never did it to my boss and my colleague. Is she testing me out or what?