This doesn't seem like an "obscure" law to me. In fact if this is a hobby of yours I would expect you to understand that it's not legal in a number of places.
It was an honest mistake, especially for someone who rarely travels.
It could happen to anyone in a country where possessing lock picks is not a criminal act. For example, your sibling might get you some picks in credit card form factor one year for Christmas. You put them in your wallet and forget about them. You travel a bit within the USA and nobody cares. Then years later you travel to Japan and are whisked away to jail because of a thing you forgot about in your wallet. The Japanese don't understand why an innocent civilian would ever have such a thing; therefore you must be a nefarious criminal.
It could happen to anyone in a country where possessing lock picks is not a criminal act. For example, your sibling might get you some picks in credit card form factor one year for Christmas. You put them in your wallet and forget about them. You travel a bit within the USA and nobody cares. Then years later you travel to Japan and are whisked away to jail because of a thing you forgot about in your wallet. The Japanese don't understand why an innocent civilian would ever have such a thing; therefore you must be a nefarious criminal.