OK, let's put this "business at the Mull" argument to bed shall we? I will agree that maybe the crew wanted to make an unplanned landing at your uber-secret LZ; like many other things, impossible to prove.
Take the planned landing/approach scenario. Why on earth would the passengers have had any interest in a demonstration of such a capability? They were on their way to a conference and had only one thing on their mind; a demonstration of an allegedly secret SH capability that those particular people would have had absolutely no interest in is nonsense. Secondly, why would the det unilaterally add such a landing to the task? All they would have wanted to do was get their passengers to destination and then go home (or stay over). There was a tenuous argument about crew-duty time; again fantasy (not saying that was your argument).
The very thought of that simple bus run task encompassing another, 'black' procedure is just too off the wall.
Head in the sand? I think not. More like a bit of myopia on your behalf.
PS. I genuinely don't have any knowledge of any FPL left behind. If I had filed one the route would have read DCT, nothing more.