"This smells to me like a myth spread by prison staff to explain away false positives and/or discourage drug smuggling." This. There are no super scanners that

"This smells to me like a myth spread by prison staff to explain away false positives and/or discourage drug smuggling." This. There are no super scanners that detect drug reside in prisons or jails. There are occasionally dogs though. Dogs will catch if you handled old school drugs (coke,heroin,etc). If a dog alerts on you - the guards know something is up. But that doesn't mean there is enough evidence on you for an arrest. Often jails/prisons will round up a shift of guards and let the dogs smell around - this catches a few bad guards occasionally. Recently, things have changed. Right now the big issue in jails/prisons is fentanyl - it is difficult to train k9s on fentanyl due to it's handling - and there are not enough k9s or departments that can do it (since it is so dangerous). Good luck finding 25 micrograms in mail or on visitors.