Being the inquisitive type, here's my research on the subject:
In this picture, I've opened five textures. Four are from STALKER, one from HL2. In the bottom left, we have HL2's Water texture normal map. Beside it, is a water texture normal map from STALKER, the file is called water_normal_dudv0.dds beside that is the STALKER water_dudv0.dds file (which looks like a frame from an animated bump map, as there are 28 of these dudv files in the textures\water directory and they play out as an animation). These dudv textures are the same as the HL2 water texture. There are also tons of other water textures in the Stalker textures\water directory.
In the top left of the image, is a texture from STALKER and another on the top right. These two images are the water I've seen in-game. They also have their own normal and bump maps, and animations. Replacing the water_dudv files from the directory with completely red images did not affect my game.
EDIT: Also while digging through the script and config files, so far I have found no place that references the water_dudv files anywhere, only the other original water textures. So it would appear that I am correct.
The STALKER game directories are very cluttered. There is leftover stuff, old stuff, stuff that's not used, etc. It's very possible they were using HL2 water textures to test shaders out, and some Doom 3 textures to test certain things, but they forgot to remove them before the game went Gold, which is what my conclusion would be. For each "stolen" texture, there is another that is the same thing, but obviously an original work. At least as far as I can tell. So while textures from other games may reside on the disc, I do not believe they are actually being used in-game.
I guess the other possibility/conspiracy theory would be that HL2 and DOOM 3 stole STALKER's stuff, since STALKER was being created before either HL2 or DOOM 3, but I think the first conclusion makes more sense.
