Nobody would WANT to come forward and admit they were stupid enough to be totally distracted by a new coupon. Plus, if you weren't too smart, and the kidnapper was just a little brighter than you, and the woman being kidnapped was a bit smarter than him, I see how things would go down this way...
The guy steps a way for a second, and the woman wants to try and leave a trail by having this lady call the cops as they walk out so hopefully they can tail the car.
But if she lets the guy know what's happening she's now just luggage that can be dropped off somewhere (like the nearest lake) while the guy tries to get to Canada and fly to Cuba or some other non-extradition rathole.
So, once the dumbass clerk actually SAYS the word kidnap in front of the man (a very dangerous move), the smarter kidnapee says "Oh, know I was just joking, he's my husband and kidnapped me for dinner."
That will make the not-as-smart kidnapper think "Oh, she must have appeared nervous to the clerk. But now she's done the smart thing and the clerk will forget about it." I'm assuming he didn't realize the clerk was ACTUALLY tipped off.
So, as they walk out, the kidnapper is thinking the clerk has been reassured, and the kidnapee is hoping that the clerk still realized something was up and will contact someone immediately. Reality lies somewhere in between the hopes of the two.
I'm not saying that was Laci Peterson, I'm just saying that of all the stories that could be told be clerks, this is not all that unbelievable if one imagines the scenario being played out in front of them.
Easy for me to say, huh?