You might want to check for following :
Firewall logs : Source IP/MAC address (Logs for given time when you suspect torrent/remote connection)
--> what kind of router or firewall you are running : Usually Soho's have logging disabled to reduce CPU usage - I am unsure in your case if router is set to log activities.
--> Try going in for static ip on this windows pc - just in case you have more then one pc.
as pointed earlier - Scheduler - Try Scheduling for something like this
Create a file in notepad and save it as watch.vbs
Set shell = CreateObject("WScript.Shell")
shell.LogEvent 3, "I was used last night at x hours"
Run Scheduler and schedule this task for hours you suspect usage.
Event Log - For any usage related tell tale tracks.
If you have too much time on hands - try searching for files created during those times for any tell tale information.
Last - what if you take out power chord and sleep peacefully ;)