Would it make any difference if the security staff had effectively mounted a "sting" operation to entrap the person? Would that be illegal making their evidence inadmissible?
The security staff may have been incentivised or targeted to discover drugs. The way my son was detained suggests he was entrapped. There was a torch signal and the security staff moved in.
He was sold the drugs at the festival and if that was orchestrated by the security staff that would explain why he was targeted.