I’ve got the alarm excessive engine speed on my Merc 600sci at about 4400rpm, does anyone know what this alarm means?

I’m going to say it’s not sensing being in gear

^^^Agree, Look on the shift plate to see if there is a switch. If there is disconnect it and see if it happens again. Disconnecting it will not set a code or do anything else

Should add; I'm not real familiar with your motor, but none of the Merc's will have any issues doing the same