If, as we understand, the mechanic has checked the battery and also the alternator output correctly then we would consider (as should the mechanic) that you have a parasitic drain. This means that after you have switched the engine off and locked the car, there is an electrical circuit still live and as such is draining the battery. It maybe the light in the boot or a interior light or another circuit. Ask your mechanic to carry out a parasitic drain as they should be able to identify where the problem lies.