That happened to me too!
The only thing that gave it away was it had the car's name embroidered on the floor mats....
The weird thing is that I noticed that the fuel door was not completely closed so I tried to close it and noticed that the spring inside wasn't latched properly so I fixed it!! (It only took about 30 seconds)
But when you are at home thinking about it your heart skips a few beats when you suddenly become aware of the many ways in which this would have gone so wrong! The owner shows up and what do you say?
When it happened to me some years ago I had walked out of a store nursing a huge headache. my car had gone for a major service and my cousin had lent me her second car. I walked up to a car that looked like mine and put the key in the door and...it opened! (I don't think it was locked). I put my groceries on the passenger seat closed the door and then a few questions popped into my head:
1. Why am I having to adjust my seat?
2. Whats this maroon handkerchief doing on the dashboard?
3. What happened to my radio? What is this junk?
4. What the....? This is not my car?
I quickly exited the vehicle and tried to walk to my car in a calm manner. It was when I got home that I asked myself how would I have convincingly answered the owner if he or she found me in their car?