You could get this on your feet by simply walking across a field where fireworks had been set off. I would be interested to know how the guy managed to drift off into the crowd while his shoe was being examined. It would seem that a normal guy would be required to stand there until the results came in, otherwise what's the point of the test? And an innocent guy would stand there, maybe a little sullenly, until the test was done. If he ran, that is suggestive.

More details on the explosive would be helpful. It takes a sophisticated explosive to fit in a shoe yet do damage, and I don't think gunpowder fills the bill.

On the other hand, I've seen women wearing shoes that you could fit a low-yield nuclear device into the soles.