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Back in the day when I was in a position to research such things, it nearly always turned out to be someone within a small company (usually in a 'marketing-like' function) taking it upon themselves to advertise the company without bothering to mention it to anyone else.
A possibility in your case?
I've also heard of sites about an area of heavy 'fan' interest being advertised using AdWords by 'fans' of that interest area who like a site (that is not their own) enough to pay a modest amount to send traffic to it.
- bid jamming (trapping)
- making "enemies" to "the site"
- masking who is who, and who is doing something
- who knows what else in order to do something unethical
I believe that, as long as you can prove the ownership of the site, you can request the removal of ad(s).
Even without explicit policy that you have to won a site in order to run ads for it, Google can still forward your request to the one that is doing it.