"out-of-band exfiltration"

the TAO master spoke of "out-of-band exfiltration" of data at high risk of discovery, one thing an alert sysadmin might observe and react to. has anyone seen a summary of the exfiltration methods identified in leaks/other so far? this would have to span everything from Google Voice Search as covert channel, to retroreflector enabled signal recovery. pointers? best regards,