Devices known as "skimmers" are sometimes inserted in ATM machines to enable crooks to obtain PIN numbers for credit and debit cards.
But for the first time, Transit Police have issued a warning after discovering skimmers at two Compass vending machines on July 8.
One was placed that day at Vancouver International Airport Station; the second was at Vancouver City Centre Station for an undetermined amount of time.
Vancouver City Centre Station is the busiest stop along the Canada Line.
Anyone who used Compass vending machines at those locations on July 8 is being asked to check with their financial institutions to ensure that everything is okay with their cards.