"The first lawsuit against Frontex by a civil society organisation was not successful: the European Court in Luxembourg, following a joint lawsuit by freedom of information activists Luisa Izuzquiza and Arne Semsrott, decided that the European Border Police do not have to disclose any information about their ships in connection with operations at the EU's external borders."
General Court Rules on Frontex: Less Transparency at EU Borders (Frag Den Staat, link):
“Frontex itself had published some of its vessels’ names on Twitter, thus showing that there can’t be a harm to public security. The information in dispute is also regularly published by other EU naval missions in the Mediterranean on a regular basis. It is not comprehensible that Frontex shall not be ordered to disclose these details, and to comply with a basic standard of transparency.”
Judgment: Case T-31/18: Izuzquiza and Semsrott v European Border and Coast Guard Agency (pdf)