Walter,
As someone who has operated and instructed on the RAF Chinook HC2 and not just gleaned information from other uninformed opinions I'm confident that there is no embaressment on my side. The CDBs can only be moved by the pilots. They can be used to display cross track information from a variety of sources including a navigation computer, but are often used just to display to the Handling Pilot a desired heading set by the Non Handling Pilot. The reasons the CDB in this case was indicating what it was will never be known but it cannot have been 'dragged' around there by an external influence. If the HP/NHP had reason to doubt the nav computer they may have been looking for a known point to 'zero' it on, please note the use of the phrase 'may have'. The only other info the CDB can work on are TACAN or VOR indications, neither of which were situated on the Mull that day, at least they weren't when I was.