In brane world string models with large extra dimensions, there are processes where fermion and antifermion (or two gluons) can annihilate producing a light particle (e.g. gluon) carrying transverse momentum and a Kaluza-Klein graviton or an excited closed string that propagates in the extra dimensions. In high energy colliders, this process gives a missing-momentum signature. We compute the total cross section for this process within the context of type II superstring theory in the presence of a D-brane. This includes all missing-energy sources for this string-theory model up to s = 8M(s)(2), and it can be used to put new limits on the string scale M-s.