Explanation according to today's Times
The British sailors and marines being held by Iran were ambushed at their most vulnerable moment, while climbing down the ladder of a merchant ship and trying to get into their bobbing inflatables.
Out of sight of their warship and without any helicopter cover, their only link to their commanders was a communications device beaming their position by satellite.
That went dead as they were captured. One theory is that it was thrown overboard to prevent the Iranians getting hold of the equipment and the information it contained.