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Hi Alex & All,
For the most part, almost certainly a data artefact arising from the digitisation of the original plates, or the plates taken at Palomar, ESO Chile and Siding Spring. Some would be small solar-system bodies (asteroids etc) that have since moved. Some would be uncatalogued variable stars caught in a flare-state or in a brightness dip. I suspect ultimately that very, very few (a few handfuls at most) would ultimately have no readily identifiable explanation. That doesn't mean an explanation doesn't exist, it's just that we haven't identified it yet.
To be definitely declared "missing" I would want a comparison with the actual plates rather than the digitised versions.
Best,
L.
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