  Context: Gaia DR3 time series data may contain spurious signals related to
the time-dependent scan angle. Aims: We aim to explain the origin of scan-angle
dependent signals and how they can lead to spurious periods, provide statistics
to identify them in the data, and suggest how to deal with them in Gaia DR3
data and in future releases. Methods: Using real Gaia data, alongside numerical
and analytical models, we visualise and explain the features observed in the
data. Results: We demonstrated with Gaia data that source structure
(multiplicity or extendedness) or pollution from close-by bright objects can
cause biases in the image parameter determination from which photometric,
astrometric and (indirectly) radial velocity time series are derived. These
biases are a function of the time-dependent scan direction of the instrument
and thus can introduce scan-angle dependent signals, which in turn can result
in specific spurious periodic signals. Numerical simulations qualitatively
reproduce the general structure observed in the spurious period and spatial
distribution of photometry and astrometry. A variety of statistics allows for
identification of affected sources. Conclusions: The origin of the scan-angle
dependent signals and subsequent spurious periods is well-understood and is in
majority caused by fixed-orientation optical pairs with separation <0.5"
(amongst which binaries with P>>5y) and (cores of) distant galaxies. Though the
majority of sources with affected derived parameters have been filtered out
from the Gaia archive, there remain Gaia DR3 data that should be treated with
care (e.g. gaia_source was untouched). Finally, the various statistics
discussed in the paper can not only be used to identify and filter affected
sources, but alternatively reveal new information about them not available
through other means, especially in terms of binarity on sub-arcsecond scale.
