  Extreme temperature contrasts between the day and nightside of ultra-hot
Jupiters result in significantly asymmetric atmospheres, with a large expansion
occurring over a small range of longitude around the terminator. Over the
course of a transit, WASP-76b rotates by about 30 degree, changing the
observable part of the atmosphere and invoking variations in the appearance of
its constituents. As recently reported, this results in time-variable effects
in the neutral iron signal, which are amplified by its possible condensation on
the nightside. Here, we study the presence of molecular signals during a
transit of WASP-76b observed with the CARMENES spectrograph and compare the
contributions from this planet's morning and evening terminators. The results
are somewhat puzzling, with formal detections of water vapor (5.5$\sigma$) and
hydrogen cyanide (5.2$\sigma$) but at significantly different positions in the
K$_p$-V$_{sys}$ diagram, with a blueshift of -14.3 $\pm$ 2.6 km/s and a
redshift of $+$20.8 $^{+7.8}_{-3.9}$ km/s respectively, and a higher K$_p$ than
expected. The H$_2$O signal also appears stronger later on in the transit, in
contrast to that of HCN, which seems stronger early on. We tentatively explain
this by silicate clouds forming and raining out on the nightside, partially
removing oxygen from the upper atmosphere. For C/O values between 0.7 and 1,
this leads to the formation of HCN at the morning limb. At the evening
terminator, with the sequestered oxygen being returned to the gas phase due to
evaporation, these C/O values lead to formation of H$_2$O instead of HCN. If
confirmed, these results indicate that individual molecules trace different
parts of the atmosphere, as well as nightside condensation, allowing spatial
characterization. As these results are based on a single transit, we advocate
that more data are needed to confirm them and further explore these scenarios.
