They'll have paid for it so if you ask then for the money they'll be paying twice. That would be a really rude expectation. Personally I wouldn't mention it as it might embarrass them.
They may not have paid for it - whether they just took it off the rack and pocketed it, without realising they have to be activated, or - possibly more likely - they did forget to activate it.
That said, I may be wrong here, but iirc, the last time I bought a gift card from a supermarket, I asked the assistant, and she told me that it was automatically activated once it had been scanned and payment successfully received.
If that's the case across the board, it does look like it's either been nicked (whether deliberately or if it fell down the side of the trolley and she forgot to scan it) or used accidentally. Might somebody have given her an identical card that she used, and then she accidentally gave her used one as a present by mistake, instead of the new one she'd bought?
Does anybody know: does it come up differently at the till, when you try and spend it, if it hasn't been activated, as opposed to it was activated but subsequently has all been spent?