FAR 25.672 has the basic airworthiness certification requirements. They include an appropriate warning that must alert the pilots to failures.
AFAIK, in practice that extends to the autopilot disconnect alarm sounding EVERY time the autopilot is disconnected, intentionally or otherwise. That is why the second press of the disconnect button is required to silence the alarm. if the alarm did NOT sound in your case, I believe that would be a grounding discrepancy.