Hello,.
Just for you to understand,when you pour oil in water, they don't mix and oil floats right.. same way in your eyes the oil can float and move around. This is emulsification. We are only guessing here,as I have not seen your eyes and examined..
In your case, you've have two vitrectomies, so that's the most probable cause.
Next Steps
if your ophthalmologist has examined your retina, then you do not have to worry about those shadows. keep regularly following up with your doctor.
Health Tips
if sudden areas of blackness develop,instead of the shadows,if you develop pain, redness etc then also immediately visit your doctor.