  In this note we revisit the notions of transverse density bundle and of
modular classes of Lie algebroids and Lie groupoids; in particular, we point
out that one should use the transverse density bundle
$\mathcal{D}_{A}^{\textrm{tr}}$ instead of $Q_A$, which is the representation
that is commonly used when talking about modular classes. One of the reasons
for this is that, as we will see, $Q_A$ is not really an object associated to
the stack presented by a Lie groupoid (in general, it is not a representation
of the groupoid!).
