I don't know what SEMTEX you think smells like Marzipan, Semtex is completely odourless, and invisible to x ray, the same as the US C4 is, though that has a smell.
The explosive that smells like marzipan is Sodium Nitro Benzene, favoured by the IRA, though Semtex, or SENTIN(the village where it was inveted) to use it's correct name is always blamed associated.
The lack of smell makes it so dangerous as bomb sniffer dogs cannot detect it, invented in 1966 by Stanislav Brebera(Czech) to clear land mines, it is a mix of RDX Cyclonite and Petn Penaerythrite Tetranitrate, it has a indefinite shelf life.
Just 12 ounces took down Pan Am flight 103 over Lockerbie, totally undetected.
The inventor now teaches at Pardubice Uni, a very unassuming man, who doesn't even own a car.
Some of the above was gleaned from a friend who had just done this as a subject, he's in bomb disposal(army), he's a right swat anyway!!
tabetha