I'd ask for more details. Does he have the gun drawn, in his hand? Is he waving it around?
If they're oblivious, I'd go watch through a window or from behind cover if possible. If it's just a dude in the parking lot with a gun on his hip, I'm not going to worry about it. If he's waving it around, I'm calling 911 and heading out the back of the store. I wouldn't confront unless it was a simple case of a CCW'er "printing".
Just my under-inflated $.02
How did it play out?