Kate loses Aaron in grocery store, woman resembling Claire finds him

After Kate leaves the others at slip 23, Aaron wants some milk. Kate stops at Tim's Grocery Store to get him some milk but he decides he wants a juice box. When Kate stops because Jack is calling her phone, she loses Aaron. She rushes through the store looking for him and as she's telling the manager to seal off the store, she sees Aaron. He is being led to the front of the store by a woman who looks like Claire from behind. Kate freaks out and grabs Aaron. The woman tells her that she found Aaron in the fruit section was taking him to make an announcement. Kate is freaked out and barely says thanks.

Kate at this point is incredibly paranoid about someone taking Aaron. This event seems to be the precursor to her leaving Aaron and going back to the island.