Thanks Eugr - I had seen that but was hoping for more specific knowledge from a forum user here. I'm toying gently with the idea that they may have seen 'lights' in the dip from one of the industrial units there or even some floods up on the light a/c park - just another possibility. Also, assuming correctly set altimeters and someone looking in, they would have been seen 'below threshold' I think.
Korn - thanks for the cross-section. Quite thought provoking. Can anyone pinpoint the '2.5 mtr' tree on the aerial view of the site?