Feeling like an asshole?

I've had my 2000 Chevy Impala for about six years now, given as a graduation gift from my excellent parents, and it's got these daytime running lights. When you turn the car off, the lights stay on for maybe 10-15 more seconds before shutting off automatically. No big deal, right? I could probably turn the setting off but I kinda like it, especially if you're in a weird place at night. It doesn't happen very often, but once every few weeks I'll park the car somewhere at night and go walking into, say, the grocery store and some kind person will inform me that I've left my lights on. My response used to be a simple, "Oh they turn off automatically in a second, but thanks," and it never fails that the person gives me this rebuffed look of like "Sheesh, I was just trying to help, dickhead." And I don't get it! Seriously, this happens every single time someone let's me know the lights are on, and I really don't think I've acted like an asshole - so I always wonder what's going through these folks' heads. So now I've amended the end of my response to include "but I really appreciate that you were looking after me," which I think just makes me sound condescending. So I can't really win. I'm almost to the point where I'm gonna just go, "oh shit, thanks so much!" and run back to the car and fake turning the lights off.