My mother's Nectar card was stolen by the cashier in Sainsbury's about 12 years ago.

The cashier was taking cards with a high points balance (she had the equivalent of over £100 on the card), and swapping them with a new, unregistered card.

We reported it to the store and they could see this happening on CCTV, so the cashier was fired. They gave my mother a Sainsbury's gift card for the monetary value stolen, plus a goodwill gesture.

Personally, my lesson learned from all of this is to spend loyalty points/rewards ASAP. While there are some "double up" offers every year that reward saving up, these are less common than they used to be. I think Sainsbury's ran a trial last year where only some people could use the "double up" promotion.