I saw this ghost image in our neighbours window (middle of the frame in dark part of the window). At first I thought it was a smudge on my glasses, then a smudge on the window, but it was neither. In person it appeared to hover in the space between me and the neighbour's window. Not sure how I'd make a diagram of what the light is doing here but it's obviously something to do with light from the white bag doing something with the double glazing.

Source: https://youtu.be/x-t3oHSDnAw

I hope your neighbour does not frequent Metabunk!

Yeah, it's a pretty standard offset reflection. A small amount of light is always reflected from glass (more when at an angle). The reflection you see has bounced off two sheets of glass, so it's doubly diminished, and only the bright bits against a dark background are visible. Essentially a lens flare if your lens was just two flat sheets of glass.

Similar examples with the sun.
https://www.metabunk.org/threads/two-suns.12976/