Re: I'm assuming
IIRC it was liquid hand soap of the type commonly used in public toilets that gave a false positive for Semtex in a high profile case.
I believe Semtex, being a trade name for C-4, has an almond smell artificially introduced to it - just as the UK's mains gas has an added smell. I might be wrong though, an almond smell might be inherent to it (cyanide?), and this subject I'm inclined not to Google it. I would like to play with shaped charge explosives though as a way of cutting metal in the workshop... the hacksaw makes my arm tired. And damn it, I like things that go Bang!