I've had random plants grow from Black Gold, FFOF, Happy Frog, various brands of coco, etc... You'll find random seeds in just about everything. The quantities of ingredients they use take a significant amount of space and are typically stored outside. That allows wind and animal born seeds to contaminate the product. Lower quality potting soils may have a higher amount of seed contamination but it's not something to use as a metric with regards to quality.
Time segment starting at 3:40 gives a good idea as to how random seeds can enter the soil mix.