I have a 1998 3000SCR with twin 5.0L engines and since I bought it 6 years ago, the alarm has been going off sporadically. I checked the three sensors on both engines and everything appears fine. To this day, I haven't resolved the problem. Talking to a Merc technician, he said that a wire could be shorting somewhere. So for now, I just disconnected the buzzers!! I KNOW, I KNOW...I shouldn't do that but I never found the problem, the gauges don't indicate a problem (water temp and oil pressure are OK) and I keep my lube reservoir filled. So, after a while, the alarm really gets on your nerves.
When I have more time, I'll get back into searching for the problem. Afterall, the alarms are nice to have if they work!