Ethical Dilemma

It's a pretty trivial but here's my dilemma:

I was running late for my clinical this morning and so I decided to park at the hospital visitors' lot although students aren't really allowed to do so before 4pm. It takes about 15 minutes to get from the student parking lots via shuttle bus and I figured that it would cost me less than $10 to park for the day and so it was worth not being late for clinical.

Anyway, when I was leaving, the lady at the booth starting questioning me. She asked me if I had gone to see a patient. I thought she was on to the fact that I wasn't suppose to be parked there, so I said yes. Then, she asked me the patient's name like she didn't believe me. So, I made up a name and said, "Smith." Then, she asked me the patient's room number like she really didn't believe me. So, I made up a room number. "Are you sure?" she asked as she jots something down on a piece of paper. Of course, I was sure. Can I have my change and get out of here? The questioning was making me very uncomfortable. I was really starting to feel guilty about do something that was pretty trivial. I mean, who would really want to pay $8.25 a day to park when you could do it for free elsewhere?

Back to the story, she hands me what I thought was $2 and a receipt and let's me go. I was really due back only $1.75 but I figured maybe she didn't have any quarters. When I get home, I realized that she gave me $7 dollars back. The receipt was actually a five. At first, I thought she gave me change for a twenty and not a ten, but the math didn't add up. Then I realized that they must give a discount to MDs who come in for short patient visits. She must have thought I was a resident or something. A Chinese man with glasses in Charleston near the hospital must be a doctor. Seriously, I didn't even have my ID badge on. But it's definitely better than being mistaken for William Hung and that's happened a few times here already.

So that's my dilemma. I accidently ripped off a little old lady for $5.25. The question is should I do anything about it? On one hand, it was an accident and parking is exorbitantly priced. On the other hand, it's not my money and it was a little old lady.