Just finished fulgrim yesterday, and I was thinking, there are a few loose ends in Fulgrim that could lead to Tarvitz, and Vipus coming back. When Eidolon asks Lucius if he saw tarvitz die, Lucius says no, but there's no way he could have escaped. That in itself is nothing, but when asked about Ancient Rylanor (the dreadnought) Lucius says he saw nothing of him after the virus bombing, as he was assigned to guard something. Those two together could add up to something, as whatever rylanor was guarding clearly was something important if they were willing to pull their only vehicle off the front lines and have him sit around doing nothing.
My hunch is it's an underground hanger, with some stormbirds or army transports in it. (I'm assuming this was Isstvanian surplus, left behind by the imperials, which is why Horus missed it. Since thunderhawks were new, they wouldn't have been left to rot on a backwater planet that was taken twenty years prior.)
I know galaxy in flames says that the precentor's palace is bombed again, but the surviving loyalists are in the warsinger's temple, and nothing is said of their death, other than "the battle for isstvan III was over". No mention of roofs crashing down and crushing astartes, nothing like that. So, it's possible that the warsinger's temple led to an underground hanger filled with ships, and these final hundred men were able to escape, as galaxy in flames says they only bombed the city. If they flew low, they would be able to escape the fleet's sensors (even if the sensors did pick them up, there's debris flying all over the place down there and it would be very difficult to discern flying rocks from a stormbird, at least from orbit) and land somewhere else on the planet, waiting until the fleet left and then figure out a way to escape the planet.
What do ya think?