  Aims. To describe two potential options for the Source Environment Analysis
pipeline, SEAPipe, for the Gaia mission. This pipeline will enable the
discovery of sources which are new to Gaia, in the sense that they were not
found by the on-board detection algorithm. These additional sources
(secondaries) are discoverable in the vicinity of those Gaia sources
(primaries) that were found by the on-board detection.
  Methods. The main algorithmic steps required are described; the 2-dimensional
image reconstruction of 1-dimensional transit data, the analysis of these
images to find the additional sources present, and the determination of the
mean positions, proper motions, parallaxes and brightness of these sources.
Additionally, the Monte Carlo simulations used to characterise the performance
of the pipelines are described.
  Results. The performance of the two options for SEAPipe, the vanilla and
image-subtraction versions, are compared. Their selection functions are
computed in terms of the magnitude of the secondary sources and their angular
separations from their corresponding primary source. The completeness and
purity of the resultant catalogue of secondary sources as found by each of the
pipelines, given the expected magnitude distribution of the primary sources and
the magnitude and angular separation distributions of the secondary sources, is
also presented. The image-subtraction pipeline is shown to out-perform the
vanilla pipeline.
