I would put money on #2.
That is clearly a differential pair between A and B, referenced, no doubt, to a plane on the next layer. Between C and D appears to be a single ended signal.
That is a microstrip configuration, and if the surface plane gets too close it would change it to a coplanar waveguide with ground, which apart from changing track and gap for a given impedance, also induces (if you are not careful) propagation mode differences.
No hyperlinks - on mobile. Maybe tomorrow.
Edit: updated with simple picture.
Looking at the top picture, we have the classic differential microstrip configuration. The required clearance is beyond the scope of this post, but we do not want to couple the fields to anything other than the pair and the reference below.
If we allow the same reference to approach from the sides, we get a coplanar waveguide with ground, a very different beast indeed. I am not going to post the equations, but they are easily available with a search.