what do monitor lizards monitor?

["King, Norm R" <NKing@usi.edu>]

Yesterday I asked (9/28/99; 12:22pm):

>Now, for something from out of the blue--does anyone know why monitor
>lizards are called monitor lizards?

To which Mickey Rowe sent me a private response:

>This is from the Oxford English Dictionary's entry for "monitor":

>   5. A large lizard of the family Varanidæ, found in Africa, Asia,
>   and Australia; so called from being supposed to give warning of
>   the vicinity of crocodiles;

Then, zenlizard sent me this view:
 
>Supposedly, the derivation of the name is from their habit of hiding out
>near rodent dens and watching for potential prey items.  Don't know if
>this is factual, though.

This is getting interesting, so I thought I'd bring everyone up to date.
Maybe there are even more versions out there.

With regard to giving warning, I guess they must just get excited, or maybe
hiss.  It is my understanding that lizards don't vocalize.  But then
monitors aren't just your average lizard, are they?