Iowa Code § 192.126: Stray containers.

When any person comes into possession of a container bearing a registered mark which belongs to another whose name and address the person does not know, the person shall immediately notify the department in writing, giving the size, shape, and mark of the container. Upon receipt of shipping directions from the department the person shall at once forward the container by a common carrier, collect, to the address furnished. Milk or cream bottles need not be returned when the cost of return is greater than the market value of the bottles.