usually posted in most shops is a disclaimer about leaving personal items in a car.....

Not to be harsh, but its your fault they are missing. You should never assume that they are going to take care of your car let alone respect it, and your personal property. I would just take it for what it is, an expensive lesson. Maybe file a complaint, but in all reality they will probably refer you to some sign that is in some obscure location.
Even car washes now have that disclaimer, they are not liable for aftermarket accessories, rims and personal property.