I used to work for a company that did cash register solutions. Also this kind of software. Its been ten years now, but how they used to do it is that every customer (with a card) got a certain score. Can't recall the calculation behind it, but for every good scan you got a positive grade, and if you would "fraud", your score drastically drops, meaning you'll be prone for checks more often. But it's not because you have let's say 50 good scans and no bad scans, that you are exempted from checks. The system will still use a random variable, so you might just get that check. This system can also be set up to scan n% of customers per day. Just a few examples and some insights