Couldn't Google just create a random element id? Instead of `warning_banner` do something like `warning_banner_x3vh2QiPm1`
Anything that adds to the DOM or uses the rebindable Javascript APIs is going to be detectable, anything more on Google's part is just security through obfuscation.
The banner shouldn't be in the DOM to begin with, anything in the DOM can be removed by client side Javascript.