Technically it could be done by Cura... if you're running multiple extruders with different coloured materials and for whatever reason put a small plate was placed below using the other extruder... but that's probably not the case.
As @GregValiant said, if you share the project file we can have a look.
It's also possible the model file is somehow invalid - there's a plugin called Mesh Tools (click Marketplace at the top right) which can analyse models for errors and perform simple fixes (though sometimes it can't fix the file itself and you'll need to use another program to do that).
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GregValiant 1,351
It doesn't look like anything Cura did to insert it. It's an odd color as well. If it was Cura it should be gray indicating the brim/skirt/raft area. If it came in with the model it should be the same color as the model.
Use the "File | Save Project" command and post the 3mf file here. Somebody will take a look.
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