I don't think this is a mole planted by Google, there are plenty of other ways to figure out what's going on there besides proactively having one of your best performers leave the company.

I'm speculating, but reasonably certain Mayer went there because she was already thinking of leaving Google, after being shut out of Page's executive inner circle. Reading between the lines (and based on input of two friends who have worked with her), it wouldn't be off base to think she's going to try to outgun Google simply to prove a point -- that they shouldn't have passed her up for promotion.

Should be interesting to watch, because I think she's motivated well beyond financial considerations -- this is about payback.