Surely the Japanese army had some sort of documentation as to where a given soldier was sent, right?
Was it assumed that the holdouts didn't return home because they were killed in action? Did they not have any sort of process to confirm a soldier returned home, or to confirm he was killed?
Contact was tried; see this prior answer by /u/RealFactorRagePolice/, and also the link from /u/mikedash at the end of this answer which goes into incredibly comprehensive detail.