Unidentified object tracked by Mirage pilots

Summary

In October 2004, during a mission over France, a Mirage 4 patrol leader was alerted by his wingman about an object that had been following them since 5 a.m. The leader managed to see it as well, but the object disappeared 15 to 20 seconds later. The possibility of another fighter jet was considered, yet air traffic controllers recorded no signal on their systems. The case remains unexplained.