Slander.
Libel is published, Slander is spoken. If they put a picture of her on a sheet that said "shoplifter" and posted in on the wall (or the net), it'd be libel.
But, I don't think you'd win a case based on Slander either:
* you'd still need to show it was published to a third party (though as you say, maybe you had friends present) * They can show that they reasonably believed it to be true (the system told them so). Their wording then becomes incredibly important - did they say "you're a shoplifter" or "you've been identified by our shoplifter recognition system"
They might also try to argue that stopping shoplifting is in the public interest and that this was a statement of opinion related tot hat.
I agree though, if this happened to me, I'd definitely be sending a letter or two to dissuade them from making a similar mistake in future (I think I'd also be trying my luck with Facewatch given that their system disseminated that "information")