An electronic card connectable to a client terminal via a memory card reader slot. The electronic card comprises a card interface having at least one power supply pin set to be connected to at least one source terminal in an electronic card reader slot of a client terminal, a wireless communication unit electronically wired to be powered via the at least one power supply pin, and a signal encoder which operates the wireless communication unit to transmit a presence signal containing a unique identifier (ID) independently from any computational operation of the client terminal.