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Typically, the rules of evidence do not apply to administrative hearings of this nature. In fact, the AR involved here makes it clear that the normal exclusionary rules of evidence do not apply to the AR 15-6 investigation.
Now, there is one exception that applies here and that 3-7(f)(7) which states that bad faith unlawful searches and the evidence derived from those searches should not be used UNLESS the legal advisor to the AR 15-6 investigation or the investigating officer made a determination that the evidence would inevitably have been obtained through legal means at a later time (through subpoena most likely). In such cases, the officer can consider that evidence.
Nothing about the 15-6 accepting the evidence though would take away the accused's ability in later actual disciplinary hearings (be it court martial or separation) from again raising those objections to the evidence being used in those subsequent hearings.
The 15-6 is just an investigation, not a disciplinary action.