  (Abridged) Short-period gas giant exoplanets are susceptible to intense
atmospheric escape due to their large scale heights and strong high-energy
irradiation. This process is thought to occur ubiquitously, but to date we have
only detected direct evidence of atmospheric escape in hot Jupiters and warm
Neptunes. The paucity of cases for intermediate, Saturn-sized exoplanets at
varying levels of irradiation precludes a detailed understanding of the
underlying physics in atmospheric escape of hot gas giants. Our objectives here
are to assess the high-energy environment of the warm ($T_\mathrm{eq} = 970$ K)
Saturn WASP-29 b and search for signatures of atmospheric escape. We used
far-ultraviolet (FUV) observations from the Hubble Space Telescope to analyze
the flux time series of H I, C II, Si III, Si IV, and N V during the transit of
WASP-29 b. At 3$\sigma$ confidence, we rule out any in-transit absorption of H
Ilarger than 92% in the Lyman-$\alpha$ blue wing and 19% in the red wing. We
found an in-transit flux decrease of $39\%^{+12\%}_{-11\%}$ in the ground-state
C II emission line at 133.45 nm. But due to this signal being significantly
present in only one visit, it is difficult to attribute a planetary or stellar
origin for the ground-state C II signal. We place 3$\sigma$ absorption upper
limits of 40%, 49% and 24% for Si III, Si IV, and for excited-state C II at
133.57 nm, respectively. Low activity levels and the faint X-ray luminosity
suggest that WASP-29 is an old, inactive star. An energy-limited approximation
combined with the reconstructed EUV spectrum of the host suggests that the
planet is losing its atmosphere at a rate of $4 \times 10^9$ g s$^{-1}$. The
non-detection at Lyman-$\alpha$ could be partly explained by a low fraction of
escaping neutral hydrogen, or by the state of fast radiative blow-out we infer
from the reconstructed stellar Lyman-$\alpha$ line.
