A strange circular band in wide-angle images

Hi,

Sometimes when I shoot with my Nikon 16-35mm lens, I get a weird circular band on my images. You can see the issue marked in the below images. I have tried to tape (non-reflective black tape) the lens but the same thing still happens. It doesn't always happen but when it happens, it's been all in shallow waters.

By any chance, do you know what could cause this issue?

Cheers,

Has to be a reflection of something - looks quite slim like the edge of a lens hood or possibly a corner or edge in the dome body,  perhaps where it transitions to the extension part?

It's so strange. I've looked at all around the lens, extension tube, dome port, etc. I can't see anything remotely reflective (there must be something, but I can't pinpoint it.). 

Thanks for your feedback.