I've noticed this effect (based on the title) when I close my eyes in a very dark room. I can sometimes "see" a very faint silhouette of my arms, and sometimes the object they're holding, roughly where they should be if the lights were on, slightly darker than the eigengrau[0]. I've wondered whether part of my brain is generating an "expectance mask" (my made up term) in my visual system.

[0] https://en.wikipedia.org/wiki/Eigengrau