The TD says that "two markables should be recorded as coreferential if the text asserts them to be coreferential at ANY TIME" (Hirschman and Chinchor 1997, page 11).
finding all the NPs that might participate in coreference becomes even harder if the annotation scheme is extended to cover event coreference (noted in the "wish list" in Section 1.4 of the TD) since it is often extremely difficult to determine which events can serve as antecedents (Hirschman and Chinchor 1997): Be careful not to get the gel in your eyes.