It magically appeared one April Fool's day, and has been attributed to some of the SREs in charge of keeping web search running, but credit has never been claimed. There was a rather funny "I am spartacus" thread about the vending machine that appeared on a major email list after one of the facilities or kitchen management asked who was responsible, ostensibly so they could congratulate them.
I no longer work at Google (though I used to work downstairs from the vending machine), so it's awesome to hear it's still up and running and being stocked. I believe, at least for a while, facilities/cafe staff were officially condoning it if not restocking it. Definitely one of the more clever pranks I saw around campus.