This paper presents a passive proximity sensor using UHF radio-frequency identification (RFID) technology. The sensing operation allows detecting and counting goods or persons with no RFID tags attached. The system comprises a customized double-antenna sensor tag and a commercial reader. The design is performed at two layers. In the physical layer, a distribution network prioritizes power towards one antenna to detect presence, while allows the other antenna communicating with the reader. In the communication layer, the RFID chip (EM4325) is programmed to set an alarm when triggered by an RF event, and hence changing the tag identifier. Design directives are given and experimental sensing tests consisting of an access gate demonstrates the proposal.