At this point I do not know why you are having this problem. Are you sure that the data you want is located in the OfficeLocations property?
As a secondary thought, have you considered creating an OU for each site and moving the account into a "Site" OU? Since you can now get the DN, you can parse it to obtain the Site.
I am ckecking my AD now to see why this is being such a pain.
