Some Canadian fish keepers have been known to come down for a visit to the states and have returned home with some fish which they forgot to mention when crossing the border. Some people are just absent minded.
Some Americans have been known to hide fish in the car of people from Canada who are in the states for a visit. These crazy Americans think this a good joke. Of course they let the duped Canadian know how they were played once they get back home.
Some fish which are kept in poor conditions wanting to get out of where they are so badly that they will leap from the tank, fill a bag with water, get in it and then manage to sneak into the car of a Canadian fish keeper visiting the states because the fish know that Canadians treat their fish quite well. It has not yet been discovered how the fish make the bag get to and then get into the vehicle where they hide, only that they do this.