The diffusive hydrodynamic limit of the Boltzmann equation in the low Mach number regime is usually described by the incompressible Navier-Stokes-Fourier equations. When the density and temperature at initial time and/or the temperature on the boundary have gradients of order 1 the limiting equations (called "ghost effect equations") are different and cannot be predicted by the classical fluid theory. Proving the hydrodynamic limit under these conditions has been an open and challenging problem. In this talk I will discuss the rigorous proof of this non standard hydrodynamic behaviour for the stationary Boltzmann equation in a bounded domain with diffuse reflection boundary condition. Work in collaboration with R. Esposito, Y. Guo and Lei Wu.