I am curious about an inverse facet of facial recognition. i.e. watching the watchers. If I owned a grocery store would it be illegal for me to use facial recognition to identify undercover cops trying to bring in under-age people to buy cigarettes to bust the sales clerks for failing to properly check ID? If so what law would I be violating? Perhaps the use of a leaked government ID database?

Are there any open source projects that could be utilized for this purpose assuming I am using a legitimately acquired database? Could I use machine learning for this? e.g. this adult has driven into a police parking lot {x} days per year and now is in my store with a teen near them This would not be to evade the law but rather to heighten awareness of clerks so they do not end up with a life-long felony record for making one mistake when things get busy.