Steam turbines are sometimes operated at part-load conditions, specifically if the energy balance in power grids featuring a high share of fluctuating energy sources necessitates such operation. At such extreme operating conditions, the last stages of the low-pressure turbines are subject to highly unsteady and heavily separated flow where Rotating Instabilities (RI) may establish, resulting in fluctuations of the flow field over a wide range of low frequencies. Previously, 3D CFD simulations of RI were performed using mainly the URANS model. Simultaneously, it was suggested that the URANS approach has difficulties in accurately predicting heavily separated flows. Therefore, the more sophisticated Improved Delayed Detached Eddy Simulation (iDDES) model is applied to understand if it can capture the complex unsteady flow physics more realistically. The choice of the computational domain influences the capability to capture detailed RI mechanisms. Here, the domain of the analysed low-pressure model steam turbine includes the non-axisymmetric inlet, axial-radial diffuser and exhaust hood. As the flow is highly asymmetric, the model spans a 360° representation of all three blade rows. URANS and iDDES simulations have been performed at various low-volume flow operating points and compared to experimental data. Both models capture the overall flow physics well, however, only the iDDES model can predict additional, intricate turbulent structures in the axial gap of the last stage during the lowest volume flow operating point. Further analysis is conducted using the iDDES approach to resolve and identify RI and the underlying vortex generation mechanisms in detail, including the onset and propagation of such turbulent structures in the flow field of the last stage.