In CORBA 3.0.3 04-03-12 non_existent states that it does not throw CORBA::OBJECT_NOT_EXIST (although it can throw other exceptions). This was also mentioned on the jacorb-developer mailing list.
Fixed with 2d273f07fc9e3db6be134ea80905c7413f6b0951 - BZ953 Add fix for non_existent