Identifying a geographically nearby mobile computing device

In general, the subject matter described in this specification can be embodied in methods, systems, and program products for identifying proximity of a mobile computing device to another mobile computing device. A computing system determines that a set of mobile computing devices are geographically proximate to a first mobile computing device based on identifying that each of the mobile computing devices received a first type of signal from an emitting device for which the first mobile computing device also received the first type of signal. The computing system determines a subset of the mobile computing devices that share a physical space with the first mobile computing device based on identifying that each of the mobile computing devices emitted a second type of signal that was detected by the first mobile computing device, or detected the second type of signal that was emitted by the first mobile computing device.