They're tracking you.

It looks fairly innocuous, a metal-and-plastic square with wires coiled up like an angular snail, a lot like the anti-theft tag you’d find if you pried apart a book you’d just bought at a chain store. But it’s a Radio Frequency Identification tag, RFID for short, and each one has a tiny antenna that can broadcast information about the product, or person, to which it is attached.

Read more at Here.