From Halopedia, the Halo wiki
Contained in the S-III folder was a file called "CPOMZ", which contained celestial coordinates in the form of a 512-character alphanumeric star chart reference string. Halsey used the CPOMZ coordinate string to locate the planet Onyx which housed the SPARTAN-III program. The letters CPOMZ are also an acronym for Senior Chief Petty Officer Mendez, the man who trained the SPARTAN-IIs at a young age. This lead Halsey to believe that SCPO Mendez was somehow mixed up with Ackerson. 
In the folder were also extensive records of the SPARTAN-II program and its members, including a great deal of biomedical data. This indicated that Ackerson had knowledge of and an interest in the supposed top secret SPARTAN-II program.