Recent case law on the scope of professional secrecy for the supervisory authorities of the financial sector and on the measure of openness of their files highlights the lack of coordination among the silos of supervision and the absence of clear and uniform professional secrecy rules across the financial sector. The introduction of the Single Supervisory Mechanism (SSM) makes this situation more acute: notwithstanding a centralised system of banking supervision, different approaches may exist in respect of access to files, even when based on EU legislation. This contribution addresses the accountability of supervisory authorities and, notably the European Central Bank (ECB), from the perspective of access to supervisory files, as a prelude to possible follow-up proceedings for failing supervision. Recent judgments in the Altmann, Baumeister, Buccioni and UBS Europe cases slowly move the case law on supervisory secrecy towards more openness, long after Hillegom v Hillenius (1984). The judgments make us wonder whether the absence of legislative co-ordination and questionable drafting is being remedied by the judiciary. The variety of legislative provisions and relevant recent case law form the backdrop of our proposal to adopt a Regulation on professional secrecy for supervisory authorities in the financial sector, which would institute a single standard directly applicable across Member States and supervisory authorities.