5D sensor problem?

[vinhdn]

Hi guys,

Yesterday, I have checked my 5D classic which was bought many years ago. I took a photo with a cap. Hence the photo was supposed to be completely black. However, the result was not as being expected. Here is the photo (I crop at 100%):

The photo was taken at 1/500s, at iso 400. I still get the same phenomenon at different Iso rate.

Does anyone know what's actually going on with my 5D classic? Thanks in advance!