They are always there and making them gone will earn you a 1.07 point modifier.
The Rub the Duck Challenge is a bit on the silly side for sure – your Challenge is to assassinate the Rubber Ducks that are hidden throughout the area.
The first duck is in a birdbath on the restaurant terrace on the side of the building.
The second duck is on the patio by the hot tub.
The third and final duck is near a planter on the main roof area.
Shooting the final Duck completes the Challenge!
Assassinating a trio of Rubber Ducks