1) Someone had forgotten to check in a class (class1) and it was throwing an error in another class (class2), where it was erroring on a missing method.
2) Carried out a full project update.
3) Out-of-date class (class1) was retrieved successfully and updated in the editor.
4) Error markings were still present in class2, even though the project now compiled succesfully.
5) Error markings were only removed in class2 if I made a modification in the editor.