  The classical or quantum nature of optical spectroscopy signals is a topic
that has attracted great attention recently. Spectroscopic techniques have been
classified as quantum or classical depending on the light-source used in their
implementations. In this way, experiments performed with quantum light---such
as entangled photon pairs---have been labeled as quantum spectroscopies,
whereas those performed with coherent laser pulses are generally referred to as
classical ones. In this work, we highlight the fact that typical
laser-spectroscopy signals should sometimes be deemed quantum too, as they
contain information about the quantum vacua of the modes that interact with the
sample. Using a minimalistic model, namely frequency-integrated pump-probe
spectroscopy, we demonstrate that vacuum contributions can be expressed as a
correction term to the \emph{classical} pump-probe signals, which scales
linearly with the intensity of the pump field. Remarkably, we show that these
vacuum contributions may not be negligible and lead to the observation of
superradiance in pump-probe experiments, provided that fields interacting with
the medium are arranged in a collinear configuration.
