Answer: is the caretaker of the bag because it is mislaid property.
Explanation: Mislaid property is property that has purposefully been placed somewhere but then the true owner then forgets about it or about its whereabouts. The person who finds this property then becomes the caretaker of the property until the owner tries to retrieve it. This property is still owned by the true owner who lost it and the caretaker is entitled to return the property to the owner when the owner comes to find it.
Krystal is the true owner of her shopping bags. After placing the bags on the counter and taking all but 1 of the bags, the 1 bag she left behind becomes the mislaid property. The owner of the store becomes the caretaker of the property until Krystal realises her mistake, and comes back for the bag. The store owner is then obligated to give Krystal back her bag, as she is the true owner of that property.