  [Abridged] Cold gas flowing within the "cosmic web" is believed to be an
important source of fuel for star formation at high redshift. However, the
presence of such filamentary gas has never been observationally confirmed. In
this work, we investigate in detail whether such cold gas is detectable using
low-ionisation metal absorption lines, such as CII \lambda1334 as this
technique has a proven observational record for detecting gaseous structures.
Using a large statistical sample of galaxies from the Mare Nostrum N-body+AMR
cosmological simulation, we find that the typical covering fraction of the
dense, cold gas in 10^12 Msun haloes at z~2.5 is lower than expected (~5%). In
addition, the absorption signal by the interstellar medium of the galaxy itself
turns out to be so deep and so broad in velocity space that it completely
drowns that of the filamentary gas. A detectable signal might be obtained from
a cold filament exactly aligned with the line of sight, but this configuration
is so unlikely that it would require surveying an overwhelmingly large number
of candidate galaxies to tease it out. Finally, the predicted metallicity of
the cold gas in filaments is extremely low (\leq 0.001 Zsun). Should this
result persist when higher resolution runs are performed, it would
significantly increase the difficulty of detecting filamentary gas inflows
using metal lines. However, even if we assume that filaments are enriched to
Zsun, the absorption signal that we compute is still weak. We are therefore led
to conclude that it is extremely difficult to observationally prove or disprove
the presence of cold filaments as the favorite accretion mode of galaxies using
low-ionisation metal absorption lines. The Ly-alpha emission route looks more
promising but due to the resonant nature of the line, radiative transfer
simulations are required to fully characterize the observed signal.
