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The parking/Autopilot ultrasonic sensors were always visible on Model X, even at the reveal. The hidden ultrasonic sensors that could see through metal that Elon mentioned during the unveiling were the ones used on the Falcon Wing Doors to detect obstructions and calculate the opening arc.
The sensors for the FWDs are probably different than the parking/AP sensors because AutoPilot uses Long Range Ultrasonic Senors, and I'm assuming they don't need those for the FWDs. Here's a Model X butt shot taken at the reveal press event, and you can see ultrasonics on the rear bumper.