Whenever they don't want Congress up their ass. Four things can simultaneously be true, despite seeming contradictory:
1) Prudent opsec against nation-state adversaries dictates that you assume 0 time for them to have a tap on a device.
2) In reality, it takes >0 time, because people processes aren't instantaneous.
3) Intelligence services sometimes break the letter of the law.
4) Intelligence services usually follow the law, because it's less hassle.
