And here what I've got using uncompressed BGRA texture data:
It looks like outlines of same picture but smaller and a whole bunch (also under every smaller copy are even smaller mip-maps but that's expected, they are stored in texture data), on picture I used black background for visibility, in reality there are same alpha channel. I've compared 2 different textures with same size and they have same texture data length so it's unlikely a compression? Also, even textures compressed with DXT can have this flag and will produce mess with just plain DXT decompression. Any ideas?
EDITED:
Never mind, figured it out by disassembling. In short - for BGR/BGRA data separated in blocks, first goes block of every pixel's B color, next G, R and A if has alpha. For DXT compressed almost same, but first goes block of first colors, then block of second colors and block of indices, and DXT3/5 another set of blocks