PING

To ping someone is to notice them. It is used by intelligence operatives on a surveillance detail, usually when the target of the operation has become aware that they are being watched, which is carefully guarded against and undesirable as it prompts the target to practice counter-surveillance drills, making them difficult to follow and usually more dangerous because they are likely to alert their colleagues that they are 'blown' (exposed) and likely to bring terrorist-type activities forward in time and even to change their specific plans so as to avoid failing in their overall mission. It derives from the characteristic sound made by a submarines sonar equipment.