Card fraud comes in a variety of shapes and sizes, but recent data from two fintech companies that track it in real time revealed some statistical red flags that could prompt credit unions and other issuers to take a closer look at certain transactions.

1. It's 3 a.m. on a Tuesday. Using customer data from August 2014 to August 2015, the San Francisco-based analytics company Sift Science looked at a sample of 1.3 million transactions with U.S. shipping or billing addresses and found that criminals are more likely to commit online fraud during the workweek. It also found that 3 a.m. is the "fraudiest" time of day, regardless of time zone.

"The average nine-to-five worker usually does their shopping online on the weekends," it said in a study released this week. "For fraudsters, 'online shopping' with stolen credentials is their job."