I was looking on Ford's site and noticed that some Explorer's have a rectangular shaped opening on the passenger side of the lower valance cut out while others don't. The only round cutouts I see on the front end house the fog lights.
I think it may be where the eye or sensor for the adaptive cruise control would be located.
Do you have a Limited without the adaptive cruise? If so it may be where the camera/eye/sensor would be to detect when you are getting to close to the vehicle in front of you if you had the option. Of course this is only an uneducated guess.
I had mine come off for unknown reason. There is a fmeale thread connection of about 1/2" under the cover which the dealer described as having purpose should the vehicle need to be towed. Sounded like an odd explaination.