I just found something out accidentally. I thought that once you discovered/hired a companion, you just had to send him to one of your settlements when dismissed. Today when playing, I accidentally pressed the exit button (circle on ps4) when the settlement window came up after I dismissed Piper and to my surprise, the game said "Piper is going to Publick Occurences". So, even if its implicit, you can actually send your companions to their original locations.