From Lexicanum

Diagnostic Covens were investigative teams of the Adeptus Mechanicus.

Overview

These covens were often dispatched to investigate Forge Worlds for signs of Heresy by the Tech Priests in charge of that planet. Antigonus defiantly told the renegade priests of Chaeronia that a Diagnostic Coven would be sent from Mars to investigate signs of their heresy against the Omnissiah.

Sources

¹ Dark Adeptus (Novel) by Ben Counter