  Using numerical simulations we study the slow dynamics of a colloidal
hard-sphere fluid adsorbed in a matrix of disordered hard-sphere obstacles. We
calculate separately the contributions to the single-particle dynamic
correlation functions due to free and trapped particles. The separation is
based on a Delaunay tessellation to partition the space accessible to the
centres of fluid particles into percolating and disconnected voids. We find
that the trapping of particles into disconnected voids of the matrix is
responsible for the appearance of a nonzero long-time plateau in the
single-particle intermediate scattering functions of the full fluid. The
subdiffusive exponent $z$, obtained from the logarithmic derivative of the
mean-squared displacement, is observed to be essentially unaffected by the
motion of trapped particles: close to the percolation transition, we determined
$z \simeq 0.5$ for both the full fluid and the particles moving in the
percolating void. Notably, the same value of $z$ is found in single-file
diffusion and is also predicted by mode-coupling theory along the
diffusion-localisation line. We also reveal subtle effects of dynamic
heterogeneity in both the free and the trapped component of the fluid
particles, and discuss microscopic mechanisms that contribute to this
phenomenon.
