As far as I know, probiotics don't implant. Some biased studies misleadingly use the word 'colonize' when they have been able to show that their probiotic bacteria takes up temporary residence in bowel mucosa, compared to totally transient bacteria which pass straight through. However I am unaware of any probiotic bacteria that is detectible in stool samples 3 weeks after stopping. Some just 'hang around' longer than others.