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I think the FOB antenna is behind the rear bumper cover.
(Watch the assembly plant video - the two "stalks" with blue tape on the tip - IIRC what I was told, they are for XM, NAV, and most fob functions)
Although by my warped logic, the pictured item is in a good location for either the motion sensor or he one that determines the distance of the fob from the vehicle.
There are multiple FOB sensors so the car can determine if the fob is inside or outside the car. Not saying this is a fob sensor but it would make sense.