Everything Google has said that the bot does, plus a knowledge of coding. They aren't using a LYNX style browser to render javascript and determine the position of ads on pages. They have to be using a headless type of actual browser for that - and it's a safe bet they aren't using firefox or IE or safari.
They aren't telling us how Googlebot is coded, but it's pretty easy to deduce.
I highly doubt this is the case, what is your source?