He basically distilled the seed himself from windblown seed:

As established in the original Federal Court trial decision, Schmeiser first discovered Roundup-resistant canola in his crops in 1997.[2] He had used Roundup herbicide to clear weeds around power poles and in ditches adjacent to a public road running beside one of his fields, and noticed that some of the canola which had been sprayed had survived. Schmeiser then performed a test by applying Roundup to an additional 3 acres (12,000 m2) to 4 acres (16,000 m2) of the same field. He found that 60% of the canola plants survived. At harvest time, Schmeiser instructed a farmhand to harvest the test field. That seed was stored separately from the rest of the harvest, and used the next year to seed approximately 1,000 acres (4 km²) of canola.

So if one windblown Monsanto seed lands on my property, and I cultivate it into acres of plants over several years, I am now a huge criminal? No. Either I should sue Monsanto for trespass, or it is a case of ownership being 9/10ths and Monsanto can only cry about it if I sell the seed to other farmers. I did, after all come by it through natural means. The court decision says otherwise, that it doesn't matter how it was obtained.

Overriding the rights of the physical property owner for the protection of the intellectual property owner is the explicit purpose of the Patent Act.[citation needed] As property rights are not constitutional rights they do not override statutes such as the Patent Act.

So there you have it. If Monsanto made glyphosphate resistant wildflowers, and those plants grew on your property in Canada (or the US) via windblown/bird/insect/etc, you are in violation of patent even though you did nothing at all.

edit: Note that the genes themselves are patented. So if the glyphosphate gene cross-pollinates intentionally or accidentally, those plants are now property of Monsanto. The patent is viral.