"x mark" in dslr images

Hi, I've noticed what looks like an 'x mark' appearing at the same position in several of my dslr images; the regular shape seems to suggest it's part of the camera's internals, almost like an AF point, rather than any outside agent such as dust etc. Below is just a random crop of two stars in Aquila, greatly enlarged in order to make the annoying 'x mark' quite obvious. Any suggestions? Thanks.

can you see this image on the camera screen or just on the computer.

it looks like the focus point guide.

what camera is it?

Just on the computer Dave. It's a Canon EOS 1100D and looking at the manual here it actually seems like an inverse of the 'Flash Exposure lock button', as arrowed below:

what program are you opening the image in?
Raw or Jpeg files?

i

Microsoft Paint and jpegs.

Very strange, if you open the images with Windows Photo Viewer do you see that mark?