[1] Mirror-mode structures have been found in the solar wind at various heliocentric distances with different missions. Recently, STEREO has observed mirror-mode waves present as trains of holes and also as humps in the magnetic field magnitude. In some cases, mirror-mode trains last for very long periods of time and have been called "mirror-mode storms". We present case studies of mirror-mode storms observed in the solar wind using STEREO data in three different locations: in the downstream region of the forward shock of a stream interaction region, inside a stream interaction region far from the forward shock, and also in the ambient solar wind. To make a formal identification of the mirror mode, we determine wave characteristics using minimum variance analysis. Finally, we perform a kinetic dispersion analysis and discuss the possible origin of mirror-mode structures evaluating curves of growth for different regimes of proton temperature anisotropies in a plasma with a He component.