I don't have a Lagoon but here are some alarms that could sound on my Saba 50 with engine & generator off:
- automatic bilge pump activation - Engine control panel is energized, but engines are off - low battery state-of-charge or low battery voltage - Carbon monoxide detected - anchor alarm set on chartplotter and boat has gone outside the radius setpoint (there are numerous other alarms that can be set on the chartplotter) - AIS proximity alarm triggered - Radar proximity alarm triggered - Man Overboard button pressed on chartplotter - Digital Selective Calling message received on VHF - Chartplotter has lost communication with Autopilot
Regarding BaardJ's point number 2, I discovered I wasn't always holding the engine "kill" button long enough after engine stopped on a Lagoon 40. As mentioned, the engine was stopped, but panel still energized.
The engine control panel making noise when the engine is off is actually the low oil pressure alarm. If you hear it with a engine running you want to attend to it ASAP! I suspect the alarm the poster is getting is the bilge alarm. When the bilge pump is activated by the float switch it sounds a alarm on most boats. Usually at or near the electrical panel there is a bilge panel that will also activate a light showing which pump is running. When out sailing it’s normal to have a pump cycle on once in a while. If your sitting at a anchorage and a pump is coming on more than once a night you probably have a issue. The first thing to do is inspect the sump where the pump is located. If clear water is present taste it to see if it fresh or salt. In every case I have had on a charter boat the pump cycling on at anchor has been caused by a leak in the fresh water system. A second hint is the freshwater pump cycling on when no one is using water. Best to shut the pump off at night and underway. While sailing its often salt water and usually caused by leaking escape hatches on boats equipped with them. G