You mentioned that you were going to remove the rear bumper to look at some PDC sensors.Since you have found this upps it is easier i think to remove all six nuts in your case five undo the necessary clips etc holding it to the side. Then remove the bumper impact plate and bumper shocks all in one. Just remember to disconnect the PDC loom as well. The missing nut was attached to a stud which passes through a flange on the rear shock absorber so could easly be replaced with a similar size bolt and just welded back on. A pic of mine when i had it off replacing the standard bumper to a M sport one. Might give an idea of what things will look like hope it is of some help.
Couple of jobs done on my old E53 X5 -
2 new front axle assemblies installed (yay for MBI policy!)
New brakes all round - pads, rotors and nice shiny discs all round.
Job well done by my local garage Driscoll Motors - now to bed in the new brakes!