The others were "intelligent" signals but from human interference. There's a lot of this so the unique patterns don't occur often enough to automate. The paper

The others were "intelligent" signals but from human interference. There's a lot of this so the unique patterns don't occur often enough to automate. The paper is actually worth reading about this part[1]. They have a moderately complex pipeline that you can think of as a filter: it tries to find anomalous signals. The stage-1 autoregressor filtered 115M signals to 3M. Then they perform signal processing techniques to remove things like GPS signal contamination ("t can be seen that certain observing frequencies contain a much higher number of events compared with the others—for example, the region around 1,600 MHz. This overlaps with known RFI at the GBT site specifically from persistent GPS signals."). After this second stage they are left with 20,515 potential signals which were visually inspected. The issue here is that we don't know what a alien signal looks like so we can't just use a classifier. The pipeline can only find things it has never seen before, but it takes human judgement to decide if these signals are "alien" or more likely contamination from human sources that weren't filtered ("Regarding the nature of the rest of the events, most of them look like false positives associated with RFI signals.") [1] https://www.nature.com/articles/s41550-022-01872-z.epdf?sharing_token=t6jjoqbFXFLJH8B5_RNzEtRgN0jAjWel9jnR3ZoTv0Mkq1U55F4UpwCyo9pvCV4lj--uzspzi_o3Nto3GrgPPPK7bN8GhKil2WvNSdFgUJmpmWo-kBOlWGQDS8nBDmrm5jSNwB_Db9767cFT2RRBBvupuVMql4JeV3b9Nn2FjQw%3D https://www.nature.com/articles/s41550-022-01872-z.epdf?shar...