Hi Samantha,
Yor are correct, n_eff is useful for determining if a mode is guided or leaky. Mode profile is also important. In this case, for the third mode if you visualize all the field components E_x, E_y, E_z, and compare it to existing literature you can see that it resembles a higher-order TE mode. The overall power resides around the Si-SiO2 interface, or the edges of the waveguide because the effective index of this mode is very close to the cladding index.
You can also change the boundary conditions (BCs) between Metal and PML to check if there are any changes to rule out BCs as the cause of an unphysical mode.
Regards,
Amrita
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