I mean it makes sense in some ways for
GK to learn about the xenos anyway since they do run into them so some cross training can only be good I'd imagine. You are right, they would probably go into disguise and just pretend they are from some made up chapter or no chapter as a black shield.
The idea came to me because of the story of:
http://wh40k.lexicanum.com/wiki/Alric
and how he got there on sublight speed travel.
It flashed back to the story in the latest
GK codex:
976.M35 Eye of the Storm
A Warp storm descends on the Rorn System in the wake of the Pallid Prince and his Daemon
host. A squad of Grey Knights are trapped by the storm on the Rorn Primarex shipyard,
having destroyed the Pallid Prince and his Warp-iron vessel. Unwilling to risk discovery by
the citizens of Rorn III, and denied Warp travel by the storm, the Grey Knights set out into
the void at sub-light speeds, the battle-brothers entering stasis for their millennia-long
voyage back to Titan.
Maybe one of them went off course and they have had lots of run ins with the necrons before...
601.M41 Unwelcome Allies
Brother-Captain Ramstorn Fane tracks the fabled Bloodwulf to the world of Crytor, the
Khornate Daemon leaving a trail of burning worlds across the sector. In the ghost-filled ruins
of the dead planet, Fane and his battle-brothers corner the Daemon before a megalithic tomb
of black glass. As the Bloodwulf and his Flesh Hounds turn to face the Space Marines, the
seals on the tomb hiss open and metallic xenos warriors of march forth.
A bitter three-way combat erupts, with the Grey Knights fighting both Necrons and the
Daemons of the Blood God. Fane finally manages to land the killing blow against the
Bloodwulf as the Daemon tries to tear apart the Necron Lord. As the Daemon collapses in on
itself in a cloud of smoke and blood, the Brother-Captain readies himself for attack from the
Necrons, only to find their lord lowering his weapon in what appears to be an archaic gesture
of parley. Deciding not to risk his remaining brothers against the xenos, Fane warily signals
for them to withdraw, satisfied that their true quarry is no more.
Though yes, the time line is skewed but so is most dates in
40k and who knows, maybe after that necron encounter, the
GK decided to send one of the same guys that got back in the slow torpedo off to the deathwatch since they couldn't spare any ships for the task and the guy seemed to survive fine the last time.
Also the name was very similar to Justicar Alaric
http://wh40k.lexicanum.com/wiki/Alaric so I misread it for quite a while thinking it was the same guy.