The Windows task scheduler will run what ever you schedul it to run when you schedule it to run, it sounds to me like you are just wondering:
IF windows is running something while you are away
windows IS running something while you are away
Or atleast that is how I read your question, to answer my interpretation I don't believe there is anything that windows is doing; obviously the computer will go into some what of a 'sleep' state or "System Idle Process" if you will were it just sits there and twiddles it's thumbs.
My theory on the return to active duty delay (which most of us probably encounter) is that windows has to figure out where it left off especially if you left programs running and if they are 'heavy' programs at that it has to check all the RAM data and the exe's read HD's figure out graphics settings as it changes over from screen saver to the UI; your probably entering your password for user login so it has to check all that,
The list goes on, simply saying that even though all these sound simple (especially for a computer that has electrons flowing at the speed of light) however it still takes time; plus as the system sits idle it builds up heat heat slows the system down and so on.
Well that's my two cents worth, no official knowledge behind this one just a theory.