Here is a video I saw a while back where the constant beep started, and He found it to be the magnet in the oil tank. He gives a pretty good explanation about the oil tank magnet, and is worth a review, if you can get past His accent, I don't think He is from around Alabama. He also shows the two wires with bullet connectors that will cancel the alarm, and He reminds us of the pitfalls of going that route. The very thing He describes does happen from time to time.
Mercury 90hp ELPTO constant buzzing - YouTube