Why would just adding a product to cart create a payment with method type "unknown" and credit card of "4111--------1111"
Fixed it!

When you delete an order on the order page, the associated payment does NOT get deleted. And, once the order is deleted, there is no way to delete the payment in Drupal commerce, since you only have a view of the orders.

Then, when you create a new order, the order number you deleted can get 'reused'. So, the old payment is then associated with this new order. That is why we saw the errant payments on the new orders.

So, I fixed this issue by going into the database and manually deleting all of the errant example payments in the commerce_payment_transaction table. So far, so good…. 

Bottom line: NEVER delete an order once live. Or, minimally, always delete the order's payment FIRST, then the order.