A tracking device was fitted by police to a vehicle used by Borce Ristevski in the months after his wife disappeared.

But the device was either removed, or fell off the vehicle, sources familiar with the investigation into Karen Ristevski's disappearance have told Fairfax Media.

Karen's husband, Borce, and their daughter, Sarah, 21, spoke to media not long after her disappearance last year. Credit:Penny Stephens

Tracking devices were routinely fitted to suspects' vehicles when police suspected foul play, the sources said.

Police detectives also suspect Mr Ristevski, 52, had been advised against speaking on phones and provided with information about investigative techniques used by homicide detectives.