I second Brian.
It's not a legal requirement, but since they look like guns...
it's a Damned Good Idea.
You can buy a small gun locker from Canadian Tire for around $100, less if you find a sale. It's a long term investment that I would consider worth the money.
Second choice is an old school locker, or a wooden cabinet with a hasp on it for a padlock, or a locking trunk.
The key here is 'locking'. A lock should keep nosy people out, which should help keep the airsofter out of trouble...