I finally solved the problem. It was (as I suspected) a problem in between
relative position of light, camera and absorbing object. But now I have
another problem. Mesh supports absorption but I thought I could simulate
smoke with a mesh and now I see it is not possible. Let me explain this.

If you make an absorbing mesh, absorption will fix to the mesh but you will
not see the cool effect of noisy smoke in the edges. To see this cool
effect you have to define a density pattern (for example a spherical
density pattern) smaller than the mesh with some turbulence. But then what
you actually see is that the absorption has the same shape for a mesh, a
box or a sphere. So doesnt seem to be a good idea to sculpt a mesh with the
shape of a smoke column and then expect absorption to do the rest. It is
better to use the trick of using several spheres  (as the smoke plug-in I
have seen).

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