Most likely it would seem as if the air is distorted by some diffuse gas other than the air contains. If you vent a nitrogen gas bottle you'll see a shadow that

Most likely it would seem as if the air is distorted by some diffuse gas other than the air contains. If you vent a nitrogen gas bottle you'll see a shadow that the gas escapes, but you cannot see it with the naked eye. So, for seeing magnetic fields you'll need also to see some specific electromagnetic wavelength which would shift in the presence of a magnetic field (much like light scatters when gases move). With complex neural networks involved in an eye it's likely that those specific light receptors found make it possible to perceive magnetic fields penetrated by sunlight. Which is basically shown in the pictures.