That explosive 'sniffers' at East Midlands failed to detect either of these substances is deeply worrying. Both are not only widely used and commercially available substances, they are a proven combination and used in every electric det in the world. Lead Azide ignites rapidly from a small electric charge through a wire filament, and the heat sets off the PETN. This in turn usually sets off a larger quantity of more stable explosive such as Gelignite, but it's the high VOD (velocity of detonation) of the PETN that gives the 'kick' to the main charge. It's also widely used in det cord - a plastic covered 'rope' of PETN that looks like washing line, handy by itself for felling small trees and the like and often initiated by poking a length of slow fuse in the end. The thought that PETN can fly about the world undetected by sniffers fills me with dread. If you've ever banged-off an electric det, a little aluminium tube the diameter of a ciggie holding half a teaspoon of PETN, you'll know the danger.
C'mon boffins. This is urgent.
(Um, in case you're wondering I trained and qualified quite legitimately in this sort of stuff as a sort of early career cul-de-sac)