UPDATE.
Well, I went back in today - just over two weeks since the incident. Went straight after the gym so had my backpack. Ideally, I wouldn't have carried it but I was in a rush. I made sure not to muck around and promptly got what I wanted. Went to the check out - same girl as before. I gave her a wry smile and placed my items on the counter.
Girl: "You know you can't bring that in here."
*Groan* You're kidding me...
"No I didn't - why?"
And I didn't know. She spoke to me off the record last time - at least that was the impression I was given. She said one of the managers had their suspicions and that she had attempted to dissuade them of said suspicions. No explicit mention was made not bring my bag in. She continued:
"So we know you haven't got anything in your bag. Just put it over there by the window."
She gestured over to the walkway at the end of the checkouts, where everyone walks through. No security. Right.
Again, this was news to me. I don't ever recall seeing a backpack sitting up the front. I asked where the sign was - a natural assumption if this were standard policy. No sign. I made mention that it's never been a problem in the 8 or so years I'd been shopping there. "That's all we're asking," she whispered. Ok, so she was speaking on behalf of the store this time round and seemed a little uncomfortable having to confront me.
Having twice been put in what is a very uncomfortable situation, I went to walk out. I finished with "I assume then no one else is allowed in with a backpack then?" She shrugged her shoulders.
That is piss poor. If they had a standing policy of no backpacks in the store and I had violated said policy - I would understand. But they don't.