A "discreet investigation" means conducting a secret or private investigation without anyone knowing, in order to find out information or evidence. Full definition
Meanwhile the Commissioner of Police, CP Ahmed Iliyasu has ordered the immediate transfer of the suspects to FSARS for discreet investigation. (securitykingng.org)
He said that based on discreet investigations, the command arrested the suspects and rescued 10 other victims from the apartment. (nigeriapoliticsonline.com)
The anti-graft agency subsequently referred the petition to a special task force which swung into action by conducting discreet investigations. (punchng.com)