Transgenic plants have been found growing where no transgenic plants were sown. The most common such plants have been canola and sugar beet. Both species have wild relatives which grow as weeds. For both these species, the possibility of gene transfer to wild plants was considered during the regulatory review process and nobody could think of any reason why such gene transfer would be detrimental to the environment. The transferred gene confers resistance to the glyphosate herbicide.
A Canadian farmer, Mr. Percy Schmeiser, was sued successfully by Monsanto, for growing its patented glyphosate resistant canola without paying the company any licensing fee. Mr. Schmeiser's defense was that the crop had appeared in his field because pollen from nearby transgenic crops had fertilized his plants one year, and that he had saved seed from those canola plants for the following year's crop. Testimony in court cast doubt on his claim because the nearest field where transgenic canola had been planted was five miles from his farm, and because almost every plant in his field was the transgenic variety. The judge decided that Mr. Schmeiser knew that his plants were Monsanto's patented variety, so that it didn't matter how he obtained them.
After Starlink corn was found in the human food supply, the gene for Cry9C was found in cornfields where Starlink corn hadn't been knowingly planted. It is possible that the company supplying seeds for these farms had mixed a few kernels of Starlink in the seed sold to these farms. It is also possible, although unlikely, that the plants grown for seed were pollinated by Starlink corn grown somewhere nearby.