Thiefs are using cheap MP3 players in a credit card scam called “skimming”. The thief installs faux facades on cash machine terminals with a magnetic reader built into it. When you insert your card the magnetic strip gets read twice, once by the machine and once by the electronics in the false card slot. Its rather low tech in that the fake card reader simply records the sound of the magnetic strip on to an MP3 player. That audio data is then retrieved from the player and later deciphered by software. As you can see below each digital 1 and 0 are represented by a long audio wave verses a short one.
If you are unfamiliar with this scam you should read up on it and check out the photos of various skimming devices so you know what to look out for.