It's possible to implement something like this and you wouldn't be able to find out, as a service sitting between the network and ad servers, it could even be embedded in an innocent looking load balancer.
Why would you think something like that wouldn't exist?
They could use that to suppress funding to websites that are not in line with Google world views and boost funding to websites aligning with their views. They could also use this to help website competitors who pay for ad words. My traffic also got down once I stopped paying for ad words, but the increase of traffic I was getting from ads, was nowhere near the size of the drop after I stopped using it. Then they capped the ad sense.
