In the 1990s, Bertram, Feinberg and Mukai examined Brill–Noether loci for vector bundles of rank 2 with fixed canonical determinant, noting that the dimension was always bigger in this case than the naive expectation. We generalize their results to treat a much broader range of fixed-determinant Brill–Noether loci. The main technique is a careful study of symplectic Grassmannians and related concepts.