That is why ongoing banking supervision is the responsibility of joint supervisory teams, or JSTs for short. JSTs are headed by ECB staff but mainly comprise national supervisors. The Bundesbank, for instance, is represented in all the joint supervisory teams for German banks as well as in a number of teams for foreign banks.
This type of understanding and authority -- as well as knowledge about the behavior of key participants -- cannot be created on an ad hoc basis. It requires broad and sustained involvement in both the payment infrastructure and the operation of the banking system. Supervisory authority over the major bank participants is a necessary element.