Revealed by forumite michi.be over at the MapCore chatboards, it seems GSC World might have been using assets from DOOM 3 and Half-Life 2. Screen captures allegedly comparing the light image folders from S.T.A.L.K.E.R. to those of DOOM 3 reveal they had been modified in various ways, such as having hue or saturation adjustments, but the original filenames were kept (for example, a file with the Doom-esque name "lights_impflash.dds), referring to one of the game's enemies. id Software CEO Todd Hollshead commented they are aware of the issue and are looking into the matter, indicating the use may not have been authorized.