It's my understanding that the LO characteristics of the F-117 are widely misunderstood. The aircraft is not 'invisible' to radar, nor is it even all that Stealthy from every aspect. Turn that big flat belly towards an enemy radar, for example, and it will be detected.
That's why the mission planning system is so vital - to avoid detection, the -117 pilot has to carefully plan a mission between enemy radars (whose effective range is signifficantly reduced by the aircraft's stealth characteristics) and turns must be carefully planned to avoid 'showing' the aircraft's more visible aspects.
My understanding is that the aircraft was detected by a newly moved mobile radar, which had not been expected.