grey lines when viewing content on tablets

Hi all,

We have noticed weird grey lines appear right in the middle of the content when viewing on tablets. We can see these lines when viewing on mobile phones but at least they are situated on the side - right hand side and at the bottom. However, on tablets, these lines are right in the middle or across the content. Any idea what these lines are? There's a suggestion that they may be a result of applying a shadow to an object but we don't really use shadows...Thank you