Gray Skies is a intel document in Resistance: Retribution.
Transcript[]
I finally found solid evidence the Maquis are hoarding Gray Tech. The large engine they kept in the VTOL hangar didn't get there on accident, which means that they deliberately moved it there, with intent to modify one of their own aircraft with the alien technology. I don't know where it came from, but it was large enough to come from a Dropship or from those large things the Chimera were constructing at the Gerolstein trenches. By the looks of the engine, it's advanced enough for sub-sonic flight, which means if the Chimera launched their armada your direction, you wouldn't know until they were already on top of you.
-- Journal Entry, 03 September, 1951 -- James Grayson