Pen tool left mark on artboards, when making new artwork it still appears

I used the pencil tool to make a random shape, but now I can not remove it, and its there on new artboards or old files I open?

I deleted the actual shape it made (or thought I did) and now i've just left with the outline 

You can see below I've opened an A4 new tab and its still there, and A3 landscape tab and its still there.

I have shut illustrator down and reopened and its not dissapearing 

Any help would be appreciated!

Thank you!