> "If it hasn't exploded in all this time, why would it go off now?"
This explains why someone might be (erroneously) confident that it won't spontaneously go off, but even if that was right, saying it hasn't spontaneously gone off doesn't tell you anything about what will happen if you instead activate the mechanism. Why would they conclude it's inert from the fact that it hasn't spontaneously gone off?
Even if their understanding of explosives is wrong, they must still understand that it's an offence to have these munitions?
Also, be careful of first aid kits from that era, as they sometimes used picric acid as an antiseptic on gauze, often in contact with a metal case...
https://en.wikipedia.org/wiki/Picric_acid
> In the early 20th century, picric acid was stocked in pharmacies as an antiseptic and as a treatment for burns, malaria, herpes, and smallpox. Picric acid-soaked gauze was also commonly stocked in first aid kits from that period as a burn treatment.
On the slightly positive side, picric acid also sublimates, but I'm not sure how long it would take for evaporate from the first aid kit. On the other hand, sublimation also increases the surface area of any metal container for forming metallic picrates.