The owner of said dive RIB is on holiday and out of the country at the moment. The reason the beacon went off was that the boat had been flooded by waves breaking over the sea wall. The boat took bit of a hammering but luckily not much damage. The auto bilge pumps did their job and eventually pumped all the water out, but not before the beacon had been set off. I beleive the boat has now been moved to a different part of the marina.
Anyway, just goes to show how effective this system is. Good one Alex.