Depending on your GPU, you could grab something like MSI afterburner. When the GPU is idle (Fresh boot, desktop) you should see the GPU clocks drop very low, something like 200mhz or lower. Then try running a game, and letting the system go back to what should be idle after that (Home closed, no games running, no chrome, etc) and see what the clocks are? It could be as you think, something makes the system thing the GPU is still being used, in which case, the system will not go into screen saver or lower power mode.