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Typically the issuer as well as card processor(in case of plastic transaction) will have their own rules, using various data-points, calculating the probability of fraud. So, an ATM withdrawal till overdraft limit between 11PM-1AM in a country other than the card-user's will cause high probability. The processor might have set a rule "send a message if probability > .5". Otherwise the alert might go to an analyst, who will look at other transactions in near past and will make a call depending on his analysis.