  We compute the mean square displacement (MSD) of intruders immersed in a
freely cooling granular gas made up of smooth inelastic hard spheres. In
general, intruders and particles of the granular gas are assumed to have
different mechanical properties, implying that non-equipartition of energy must
be accounted for in the computation of the diffusion coefficient $D$. In the
hydrodynamic regime, the time decay of the granular temperature $T$ of the
cooling granular gas is known to be dictated by Haff's law; the corresponding
decay of the intruder's collision frequency entails a time decrease of the
diffusion coefficient $D$. Explicit knowledge of this time dependence allows us
to determine the MSD by integrating the corresponding diffusion equation. As in
previous studies of self-diffusion (intruders mechanically equivalent to gas
particles) and the Brownian limit (intruder's mass much larger than the grain's
mass), we find a logarithmic time dependence of the MSD as a consequence of
Haff's law. Beyond the logarithmic time growth, we find that the MSD depends on
the mechanical system parameters in a highly complex way. To explain the
observed behaviour, we analyze in detail the intruder's random walk, consisting
of ballistic displacements interrupted by anisotropic deflections caused by the
collisions with the hard spheres. We also show that the MSD can be thought of
as arising from an equivalent random walk with isotropic, uncorrelated steps.
