As you stated that you had filled the stern gland with grease, and that it was empty i am interested in why you don't know where it is. If it is safe to assume that you turned the stern tube greaser and that is remote from the stern tube, and that this required loads of turns, it would be wise to suspect that the packing has either disintegrated or that the assembly needs tightening. As others have said, if you follow the propshaft it will be at the end where it leaves the boat. If your drive is hydraulic, either follow these to the stern or trace the tube leaving the greaser. Either way, it will be situated at the centre of the stern of your craft. Then post a picture.