Stringing between holes
Consider that the string you are seeing is the nozzle wiping across the surface, especially if you are not getting stringing via your test part. As you suggest, it's not unlikely that the layer viewer is showing movement and not extrusion.
Check your z-hop settings as well as the other references in the linked page. Z-hop will drop the bed by the set amount when traveling with retraction.

The other useful reference is combing, which limits the travel of the nozzle to remain within the boundaries of the print and also removes the requirement of retraction. It may leave nozzle traces, although that reference is not in the linked page.

All images attributed to linked page.