I have searched high and low throughout the message boards and help forums of the internet, and I am left with the unfortunate reality that many other Fallout 4 players are facing on the Xbox One and otherwise. After changing to a different companion (in my case, Piper) and sending Nick Valentine to a settlement, he vanishes. He is no longer at the settlement, he does not respond to the call bell, and he is in none at any of the stereotypical places in which this glitch will cause him to be located. I have checked all my settlements, the Valentine Detective Agency, and the Publick Occurrences building, and his is nowhere to be found. In fact, it appears as if the settlement population listing on the Pip-Boy map has even gone down to reflect his absence, even though I did not relocate him a second time. In my game, I sent him to Croup Manor, but this problem can occur regardless of where he is sent, including his business headquarters. Again, this companion disappearance is a huge problem that has vexed many other gamers, as Nick Valentine has been effectively glitched into nonexistence and is now unfindable. I didn't notice until I was too far in to reload an old save, or I would have done that rather than make this post, but it appears no one has vocalized this problem yet on these forums, and this is definitely a patch-worthy bug. Thank you.