A compromised system would not readily reveal itself unless its user is sensitive to seemingly natural occurrences that aren’t natural at all. A compact disc drive that opens and closes on its own, frequent runtime errors, unexplainable sudden disappearances of files, among others indicate that there is something going on.
The problem with the many symptoms associated with being hacked is that they may be indicative of other things as well. The more usual signs like discovering emails in one’s sent items folder that were written by another, finding that files have been moved and the disappearance of desktop icons and toolbars may also have another explanation. Email bombs and viruses are clear evidence of an infiltrator’s presence but logical attacks or invasion of the network may be more difficult to detect.