Large-Eddy Simulations (LES) results are used to track flow separation zones in a splittered vaned radial diffuser near the compressor best efficiency at high rotation speed. The topology analysis reveals some closed separations at the corner of the blade and hub or shroud surface. In addition, an open-closed flow separation at the main blade pressure side is observed for the first time. The surface separation has the shape of a sheet at the blade mid-span and the associated separation line is delimited only at its downstream extremity by a trailing edge node. This flow separation process is robust to a small shift in operating condition and the separation surface remains essentially unchanged. A large flow rate variation converts the open-closed separation into a more classic closed separation. The evolution in time of the separation surface is also observed and the authors conclude to the unsteady existence of the open-closed separation.