Why Windows Authentication popup when Custom error is ON

Question

Why Windows Authentication popup when Custom error is ON when everytime i input info in a textchanged on my master page..Help me please..
I set a htmk file errorpage ERRORPAGE2.htm

Here's my code in WebConfig

<customErrors mode="RemoteOnly" defaultRedirect="ErrorPage2.htm">

   <error statusCode="403" redirect="NoAccess.htm" />

   <error statusCode="404" redirect="FileNotFound.htm" />

</customErrors>

it is working everytime error accured but when i input letter on my search texhchange in masterpage windows authentication security always prompt..how can i resolved this..