Well automatic download option is tied to Instant On I think. If you're not set to Instant On then I don't think it should be downloading anything. But if you share access to your account then people can still download stuff without you.
Id just sign out of your account when done. But I honestly dont have a console anymore! Maybe some better answers will arrive tomorow.
I dont think that the xbox will secretly download anything. Im sure you can monitor it by checking the hard drive space ect.
