  The neutral intergalactic medium above redshift 6 is opaque to ionizing
radiation, therefore one needs indirect measurements of the escape fraction of
ionizing photons from galaxies of this epoch. Low-ionization state absorption
lines are a common feature in the spectrum of galaxies, showing a diversity of
strengths and shapes. Since these lines indicate the presence of neutral gas in
front of the stars, they have been proposed to carry information on the escape
of ionizing radiation from galaxies. We study which processes are responsible
for the shape of the absorption lines, to better understand their origin. We
then explore whether the absorption lines can be used to predict the escape
fractions. Using a radiation-hydrodynamical zoom-in simulation and the
radiative transfer code RASCAS, we generate mock CII 1334 and LyB lines of a
virtual galaxy at redshift 3 as seen from many directions of observation. We
also compute the escape fraction of ionizing photons in those directions and
look for correlations between the lines and the escape fractions. We find that
the resulting mock absorption lines are comparable to observations and that the
lines and the escape fractions vary strongly depending on the direction of
observation. Gas velocity and dust always affect the absorption profile
significantly. We find no strong correlations between observable LyB or CII
1334 and the escape fraction. After correcting the continuum for attenuation by
dust to recover the intrinsic continuum, the residual flux of CII 1334
correlates well with the escape fraction for directions with a dust corrected
residual flux larger than 30%. For other directions, the relations have a
strong dispersion, and the residual flux overestimates the escape fraction for
most cases. Concerning LyB, the residual flux after dust correction does not
correlate with the escape fraction but can be used as a lower limit. (abridged)
