Find a new mechanic. With the alarm going off that means there is an active fault and should be present with a scan tool. Either he doesn't know how to use the scanner or it's not the proper scan tool.
One time I had a mercruiser that would randomly shut off, then beep. No active faults when it happened and no stored faults. Came to find out the motor had been removed for a coupler and when reinstalled the main ground for the harness/ecm was on the opposite stud from the main battery ground. Got the grounds stacked together on the same stud and issue went away.
