Could it be a remnant of something the car had passed recently? Do you by any chance pass a pillar close by when maneuvering into such a parking spot? I know I have such a place in my daily drives where, if I park right after a certain spot, it will often warn me of an obstacle (and I notice by its safety refusal to let the front door open properly for a moment). The obstacle isn't there, but it was there "on route" a few moments prior when I drove in. Tesla added this type of safety feature early in the Model X process because the sensors without such "memory" were missing obstacles in some places, so they added a "memory" that knows it just passed something close, it may still be there, so better be safe than sorry... This was because the car has door and roof sensors only in certain spots and not all over the car, so having "memory" of what it has passed recently tries to alleviate this. I get it that it can be annoying, but I've gotten over it personally. I'm not at all convinced the door setup is a good idea in the long term, but the inner 9 year-old in me likes it for the time being.