In one embodiment, a method includes, by an RF transceiver of a wireless communications device associated with a first user of an online system, transmitting an RF reference signal to each antenna of multiple antennas coupled to the transceiver. The method also includes, by a processor of the wireless communications device, determining, for each antenna of the multiple antennas, at least one characteristic of the RF reference signal reflected back from the antenna. The method further includes, by the processor of the wireless communications device, characterizing, based on the at least one determined characteristic, at least one aspect of an environment around the device to determine that an object is approaching the device. The method also includes, by the processor of the wireless communications device, retrieving from the online system, based on determining that the object is approaching the device, information associated with the first user.