White Lies

I was in a store today and overheard a woman talking on her cellphone. The thing that made me suddenly tune in on her conversation was her line "Look I'm standing at the checkout counter in a store and I really must go." I stopped mid-rack-surfing to see her crouching by a large pillar, talking into her cellphone, no where near a counter or a checkout. So why did she need to lie to the person on the telephone. Was the truth that she was in a store and unable to talk right now not a strong enough reason to end the call? Why did she need to add the "at the checkout counter" part?