In cities where Uber is illegal — of which there are still many — Greyball can identify undercover police who are trying hail Uber drivers, bust their drivers, and impound their vehicles.
What do Uber, Volkswagen and Zenefits have in common? They all used hidden code to break the law.
Quincy Larson

I wonder how they knew there were actually cops and not just any other person who worked in public institutions and his/her credit card was also linked to government agencies. But well credit card mining wasn’t the only approach that they used, I bet.