It all depends on how truly random the system is. Each checkout (or ticket, or whatever) assigned a random number between 1 and 20, with 20 meaning audit? That's non-discriminatory. But it's also not tuned for the purpose of finding shoplifters (etc).

When you start adding criteria, they are often at least correlated with discrimination. Food stamps were mentioned elsewhere. Flight history to/from a list of hostile countries for airports. The list goes on. Technically not based on things like race, but it's a paper-thin distinction in some cases.