It cannot be detected. The computer emits compromising radiation which can be reconstructed from a remote location. There is no need to ever come near the target. It can be performed from an office or a vehicle with no chance of discovery. The premise is very simple.
All electronic devices emit some low level electromagnetic radiation.
Whenever an electric current changes in voltage level it generates electromagnetic pulses that radiate invisible radio waves. Similar to the ripples caused by dropping a small rock into a quite pool of water. These electromagnetic radio waves can carry a great distance. When the electrons strike the screen they cause the pixels to fluoresce. This beam scans across the screen from top to bottom very rapidly in a repetitive manner, line by line, flashing on and off, making the screen light and dark, creating the viewed image.