Interesting. At LHR T5 this week, I had taken off my boots (they always alarm the metal hoop) and my belt. I was still sent through the fancy wave machine and STILL said that I had to have a pat down AND the metal wand.
The interesting remark from the person was this: "The system shows your lower right leg." So he made careful inspect by hand and wand. Finding nothing, he concluded, "Your trousers were hanging down and sometimes the folds of the meterial on your foot can do it."
As I had to take my belt off, it is not surprising that my trousers were drooping! On my way back from EDI, after taking the belt off - I made sure to yank my trousers up. Went through just fine.