Unfortunately, there is at least one high explosive
that I know of that is undetectable by smell or electronic sniffer, contains no nitrogen, so liberates no nitrogen oxides, and is pathetically easy to make. It can also be set off by a detonating substance (not requiring a blasting cap) which can be hidden beyond the reach of any practical search. It is a clear, odorless, colorless liquid that is safe to handle until exposed to the right substance -- then BANG!