If there was a distress signal sent two hours after the plane went off of radar, it may have been sent by the "PING" from one of the two black boxes. I never flew an Airbus, but the plane may also have an automatic activation device that sends out a distress signal under certain circumstances, like loss of altitude. It's not always so easy to see other aircraft in the vicinity. We depend on radar to locate aircraft and also if my plane gets too close to another plane, all planes (airliners) have a system called "TCAS", which stands for "Traffic Collision Avoidance System". This system will alert the pilot to either descend or climb, if another aircraft comes within 1000 feet or in some cases more, depending on speed and altitude of his plane.