I found the presence of a seam (if that's the appropriate word) while visiting "Khalaan, Old Landing" as part of the "The Forsaken Betrayer" Quest. (see screenshot)

(in-cell coordinate : 2842.107910, 4484.282227, 10064.777344)

(and there is another one, seemingly from the same mesh. Disregard the bugged minimap, that's a problem with my experimental version of OpenMW)

Update : found another one in St. Delyn Plaza. It's also a hole I'll probably have issue to get out without levitation.