If the Num Lock key active light is lit, on closing down the PC using the top bar menu option Shut Down...the light on the keyboard remains lit.
If the Num Lock key is depressed before closing down (the light extinguishes) and then closed down, all is well with the world.
The keyboard is a pretty much standard (if ancient) Logitech Internet Pro Keyboard which has worked fine on prior owned PC's running Windoze XP.
The PC is totally dedicated to Ubuntu and was installed from scratch on first use fairly recently.
No lights remain on the PC after close down. There is a not easily accessible switch underneath the machine which removes all power from the PSU.
The system specs:
Ubuntu 14.04 64 bit 3.7 GB RAM Intel Celeron (R) 847 @ 1.1 GHz x 2 Intel Sandybridge Mobile 730+GB HDD
It's no big deal, but just curious as to why this might be the case.
This CNET website says that PS2 outputs can be wired that they are always on.
Maybe time for a USB keyboard ;-)