If someone else has physical access to the PC at all then look towards them first. Don't think because your PC has password then its safe, it's really not.
If all of the comms hardware is turned off then no-one can access it remotely. Unless of course there is some scheduled task that turns the hardware back on at certain times then off again perhaps when you are at work. But that's all a bit James Bond...possible, but they would REALLY have to want what is on your PC.
By the tone of your question, you sound like you have some sort of evidence of tampering? You could always check your PC's logs.
In short...no...Looking through your window is the only obvious remote access if everything is off