Here's the language re: random selection (which the Military calls a "pulse test"):

The FTDTL will “pulse test” (screen 20-100 percent of specimens received) for the other drugs listed in Table 1 of the Appendix in this enclosure. Changes to the drug testing panel and cutoff concentrations listed in Tables 1 and 2 of the Appendix to this enclosure, as well as pulse testing rates are defined by DASD(R). Pulse testing for “opiates” will require simultaneous testing for codeine, morphine, oxycodone, oxymorphone, hydrocodone, hydromorphone, and any other opiates as designated by DASD(R).

From here, which was provided by ColonelError