Chances are the guy also forgot to pay for it too.
That's why they have a security guard to make sure no one walks out with unpaid items.
Ended up bringing it back into the store, if the guy did pay for it and realized I left it in the wagon, he will go back to the store thinking it's in his wagon or someone brought it into the store, and ask if someone brought in a case of Seltzer that he forgot to take , and lastly could very possibly be he forgot to pay for it like you said.