I'm certainly no expert but I would say both. Seeds have a way of being transported via wind, animals, vehicles etc. I recently read that 1 water hemp plant can produce between 300,000 and 5,000,000 seeds so it only takes 1 bad egg to produce a problem.
I know I've always used gly on the higher end of what's recommended. But there is always that bit of drift on the outside edge of where your spraying. I'm seeing it in only 1 of my plots, the others are clean??? I assume it won't be long before its everywhere. I was wrong in thinking it laughed at 2-4 db, it just took longer then I though to work. As of yesterday the hemp sprayed with that is curling and yellowing.
Like Mo says be aware and prepare. I'm trying to formulate a plan of attack for next year.