  We present multi-epoch VLT/SPHERE observations of the protoplanetary disk
around HD 135344B (SAO 206462). The $J$-band scattered light imagery reveal,
with high spatial resolution ($\sim$41 mas, 6.4 au), the disk surface beyond
$\sim$20 au. Temporal variations are identified in the azimuthal brightness
distributions of all epochs, presumably related to the asymmetrically shading
dust distribution in the inner disk. These shadows manifest themselves as
narrow lanes, cast by localized density enhancements, and broader features
which possibly trace the larger scale dynamics of the inner disk. We acquired
visible and near-infrared photometry which shows variations up to 10% in the
$JHK$ bands, possibly correlated with the presence of the shadows. Analysis of
archival VLTI/PIONIER $H$-band visibilities constrain the orientation of the
inner disk to $i = 18.2\deg^{+3.4}_{-4.1}$ and ${\rm PA} = 57.3\deg \pm
5.7\deg$, consistent with an alignment with the outer disk or a minor disk warp
of several degrees. The latter scenario could explain the broad,
quasi-stationary shadowing in N-NW direction in case the inclination of the
outer disk is slightly larger. The correlation between the shadowing and the
near-infrared excess is quantified with a grid of radiative transfer models.
The variability of the scattered light contrast requires extended variations in
the inner disk atmosphere ($H/r \lesssim 0.2$). Possible mechanisms that may
cause asymmetric variations in the optical depth ($\Delta\tau\lesssim1$)
through the atmosphere of the inner disk include turbulent fluctuations,
planetesimal collisions, or a dusty disk wind, possibly enhanced by a minor
disk warp. A fine temporal sampling is required to follow day-to-day changes of
the shadow patterns which may be a face-on variant of the UX Orionis
phenomenon.
