I find your reasoning that the terrorist "would never admit" anything about the real chemicals. If he used ammonium nitrate, he can easily say "I was using some fertilizer in my garden". If he used gunpowder, he can say "I was at the shooting range earlier", or "I was setting off fireworks with friends last night". If neither of those are what they found, they're still normal-sounding excuses that a person might give. He can give a long list of innocuous chemicals with those thrown in, and you won't have any way of distinguishing his story from that of an innocent person.
But if the terrorist hasn't thought this far ahead, then his excuse is probably not going to be very good. Tell a person exactly what you found, and you might catch the terrorist off guard and be able to tell that they're lying from the weird excuse, while a normal person will either say "I don't know where that would have come from", or would actually have a plausible excuse.
It's still not a very useful question even in that latter case, but it's marginally more useful if you actually name the chemical.
But if the terrorist hasn't thought this far ahead, then his excuse is probably not going to be very good. Tell a person exactly what you found, and you might catch the terrorist off guard and be able to tell that they're lying from the weird excuse, while a normal person will either say "I don't know where that would have come from", or would actually have a plausible excuse.
It's still not a very useful question even in that latter case, but it's marginally more useful if you actually name the chemical.