This could be from be a different set of cells in your eye;
$1 only sense the basic presence of certain wavelengths of light, and the signals are transmitted and processed differently.
One of the ways they confirmed their function was by finding people who had no remaining vision, but were still somehow aware of light and dark, and still maintained day/night circadian rhythms. So...yeah; shadow perception is apparently totally a thing, separate from vision.
But yeah it could also be subtle changes in air currents, temperature and echoes. Lots of subtle cues big objects have.