Try a search on TCAS.
There have been several threads about TCAS, and one in particular about target peculiarities.
I'm too dumb to figure out how to post a link, however if you also search on my user name you should find it.
Another possibility is that the traffic in question was being filtered by the radar.
E.G. Controllers may filter out certain sqawks, e.g. 7000, to declutter their display and make it manageable. However, if someone with a filtered sqawk gets lost and goes somewhere they shouldn't....
CPB
[This message has been edited by Capt Pit Bull (edited 15 October 2000).]