bndtools will look for external sources for a bundle by trying to find a bundle in the repo with the Bundle-SymbolicName(bsn) suffixed by “.source”. So if the bundle has the bsn com.acme.foo, then bndtools will look for com.acme.foo.source in the configured repos.
Also, if you manually edit the source attachment information for the Bnd Bundle Path classpath container, bndtools will persistently remember all the source attachement settings for the project. This info is stored in the file: .metadata/.plugins/bndtools.builder/<projectname>.sources where <projectname> is the name of the Eclipse project.