Hi Ryan and welcome to the Lincoln Forums. They generally do not "fall in to the rear bumper" without some help...i.e. being hit etc. They are held in with clips, so it would seem the clip on yours broke.
The bumper can be hit with enough force to break the clip and knock the sensor loose, without leaving visible damage to the bumper cover. That is the usual cause of issues like this.
Removing the rear bumper cover and replacing the clip would be the steps to fix it.
