From a conceptual viewpoint it's one thing to speculate.
Simple chlorine's usefulness in canisters is entirely different from yellow cross in grenades; or other types. Some could be used as pesticides, others would just pollute 'the barn' so much to be utterly impractical. And disarming the cap means there was practically zero knowledge for that 'idea' in say agriculture or urban pest management. But start to think less 'souvenir' but 'political activist' and even the craziest chemical might attract a crazy crackpot?
That said, some of the chemical weapons were after the war not overseen destructed by allied military but from 1918–1920 partially 'set aside' by German military and private companies, officially to be 'delaborated'.
Apart from these clandestine operations it does indeed happen to this day that dumped into the sea chemicals leak and get washed up on the shores in the form of toxic lumps. Unidentifiable for the casual tourist.
But sometimes the souvenir aspect takes over in people visiting the battlefields. Taking a grenade from an abandoned trench storage maybe somewhart 'safe'. But people still take fired but unexploded shells home today:
A farmer had called to say he had left a shell on the roadside, beside a metal pole with a yellow tip, but it is not there. ‘It seems that someone has taken it for a souvenir. There’s no other explanation. It happens,’ he says. ‘People take these things away without knowing what they are. The explosive danger is little, but the toxic danger is great.’
His colleague Geert Denolf adds that unscrupulous locals sometimes take shells from the roadside, clean them up and sell them to unsuspecting tourists in the markets of Ypres. ‘It will be a booming business during the centenary years,’ he predicts. ‘Booming’ in more senses than one, perhaps.
–– Martin Fletcher: "Lethal relics from WW1 are still emerging", Telegraph, 12 Jul 2013
And not only in France and Belgium were some hidden treasures lurking below ground. On the grounds of American University in Maryland's Spring Valley a private company dug up
a sewer line in front of a recently constructed house in a luxury development uncovered a cache of rusting munitions, including four unexploded mortar rounds and three 75-millimeter artillery shells. […] determined that the mortar rounds still had fuses in them-that they were "live" and extremely dangerous. Some of the munitions were also believed to contain mustard agent…
–– Jonathan B. Tucker (2001) Chemical Weapons: Buried in the Backyard, Bulletin of the Atomic Scientists, 57:5, 51-56, DOI: 10.1080/00963402.2001.11460494
And of course there was more where that came from. Much more. During the war they simulated Verdun battleground, killed a lot of goats with poison gas shells and explosives and after the war just filled up the simulated trenches without proper cleanup. When suspicions about this site formerly called Death Valley resurfaced in 1986 the army simply declared the area safe – just to the University, not the public – as there had been orders to remove the poison…
Now transfer this outcome from a very orderly environment in the US and apply that to the trenches system from Switzerland to the North Sea. It is still such an issue that Eurostar train line issued warnings to passengers that collecting grenades is forbidden and especially prohibited is taking them onboard the trains.
According to 2010 numbers 60 metric tons of duds are cleared annually from the Verdun battlefield by demineurs now.
Q Were there any instances where a military grade chemical weapons ended up in private hands after the WWI?
Quite a lot. One instance would be this:
–– Tom Parry: "The First World War bombs that are still killing people in France", Mirror, 14 Jul 2014.
