Arguably the Quorum don't ever go into the field and they likely seldom use Sorcery.
If you like, moenghus could have taught it some anagogics to bruise it up a bit and just hoped it never had to use sorcery whilst it was there.
I'd argue as well that the Consult's brand of Gnosis differs gently from the Mandate's and they may have suspected the skin-spy if it was seen doing sorceries, even if it was still gnostic cants/wards. The spy was at a great risk of being discovered but being within the Quorum was worth the risk.