Comment by hunter2_

Your 4th hypothesis is equivalent, not "other," to mine. But I agree that it could be anything you mentioned, as all of them have crossed my mind as possible reasons why I experience "verifying you are human" and "find the motorcycles / busses / traffic lights" more often on my residential ISP than on my cellular or work ISPs, and the unknown botnet/exit node seems most probable (by far) when considering my own traffic. I never have the 403 problem discussed here, luckily.