A Power Mac G4 computer's logic board may have a red light, which may occasionally be visible through the case when the side door is closed. The characters "DS5" are printed on the logic board next to the light.
The light may be lit whenever the computer is turned on, regardless of whether you're using it or have put it to sleep. When you open the computer for service or an upgrade, check to see if the light is lit. If so, be sure to shut down the computer before continuing. Choose Apple menu > Shut Down (Mac OS X) or Special > Shut Down (Mac OS 9).