Default skinning for vertices with no assigned vertex group I am trying to fix a problem with the FBX exporter. Two vertices in my test model don't seem to have bones/weights assigned to them. This is confirmed by printing vertex.groups to the console during export for those vertices, and by checking the vertex in the Blender UI, so it is not just an exporter or API problem.

Blender still seems to skin these vertices correctly in the UI when I move the skeleton.

Now a simple fix for the model would be to assign the vertices to appropriate vertex groups. But I am curious how Blender calculates the weights/influences for those vertices. Is it a hack based on neighbouring vertices, or does it go through bone envelopes?

A test model is at opengameart.org/content/dwarf-fixed . If you lift either boot in Blender it looks fine, if you import this into Unity the vertices that are reported as having 0 weights by the exporter are stuck to the floor. These vertices are in no group, but they still skin fine in Blender.
There is no system based on neighbouring vertices.

Most likely it is bone envelopes, you can look at the Armature modifier to see if they are enabled. If they are, disable them and see if that stops the vertices moving along with the armature.