The Augury Malifica is a heavily modified Auspex scanner, barely recognisable as the original device, crafted by the techno-seers of the Grey Knights Chapter. Strange attachments have been added, subtle alterations have been made, and seven rituals of detection were performed to consecrate the Augury. The result is a piece of equipment that can, with a reasonable degree of accuracy, detect the malignant auras of daemons in the vicinity, and even, on rare occasions, predict an imminent warp breach. 
It lies in the Deathwatch's Vaults but not with the blessing of the Grey Knights, for they are entirely unaware that the Deathwatch have it in their possession. Rather, they believe it lost to the foul hands of the Word Bearers Chaos Space Marines, and have been actively seeking its recovery for many years. It would be most detrimental to Chapter relations should the item's real location be revealed.