Question, is hiding a link or making the text with link small to the point that it will just look like an underline a type of hidden advertising?

It popped in to my mind a while ago coz I remember last year I found an underline that is actually a keyword that used a Size 1 font.  Its almost invisible to the naked eye coz it really looked like a plain line.. 

Yep, its a silly thing to do. Why would you want to anyway? All links on your site are either navigation or citations so there should be no need to disguise them. I am actually having trouble thinking of reason for doing this?

It isn't the right thing to do and maybe your site will decreased it's reputation on the web by practicing that method.

Its not matter how you show your links.