  We consider finite element discretizations of Maxwell's equations coupled
with a non-local hydrodynamic Drude model that accurately accounts for electron
motions in metallic nanostructures. Specifically, we focus on a posteriori
error estimation and mesh adaptivity, which is of particular interest since the
electromagnetic field usually exhibits strongly localized features near the
interface between metals and their surrounding media. We propose a novel
residual-based error estimator that is shown to be reliable and efficient. We
also present a set of numerical examples where the estimator drives a mesh
adaptive process. These examples highlight the quality of the proposed
estimator, and the potential computational savings offered by mesh adaptivity.
