The ones I’ve encountered were flagged, usually they have a 4 flag layout with the device in the middle and were easily identified. Problem is a critter such as a dog doesn’t identify. The fact that the dog died and the boy only suffered headaches raises some red flag. The devices that had the cartridge charge was replaced by the spring loaded M-44 back in the 60’s. The device shoots out a powdered cyanide that travels about a couple of feet into the air. The device is set off by a pulling action such as a animal would do to trying to pull the baited section out of the ground. The powder is shot into the mouth where it mixes with saliva and turns to gas and kills quickly. Usually the animal travels less than 5 yards.
I’ll go out on a limb and say the dog set the charge off and the boy probably found the dog after the incident. Had the boy set the charge off headaches would have been the least of his problems.