So from what it looks like the WCS creates a track file in TWS upon first radar sweep over target. Upon the second sweep it calculates the difference with the original track information and adds velocity information.
However, if the radar only manages to sweep once over the target before it dissappears (notched, outside scan zone, etc.), the track file will never age out, even if it's outside the scan zone, which results in a lot of "ghost" track files.
The only way to remove them is by switching to a pulse mode so the WCS drops all tracks.
How to reproduce:
In TWS modes, let radar sweep once over target, then fly so that the target is outside radar scan zone before it sweeps over it a second time. The created track with no velocity information will never age out.