I would treated the patient's own red cells with papain or ficin (whichever is used by the manufacturer to make their enzyme treated red cells), and then test them against autologous plasma. My guess (and from this far away, it is just a guess) is that they will be positive.
I think that there is a "cold" reacting auto-antibody there.
I would also suggest performing an IAT panel in tubes, bringing the reactants to 37oC before mixing them, and using isotonic saline, rather than LISS as the red cell diluent, and then washing the tests in pre-warmed isotonic saline. This should show i