Bethesda...
In EVERY! Bethesda game that I've played that has followers... They end up lost on their own adventures. Be it while they're following me, or when I send them to some place in Fallout 4.
You really need to add in a Companion Tracker, so whenever a companion is lost, we can re-locate them! My partner has spent 6 hours, in diamond city, goodneighbour, greygarden and various dungeons... Not once!, re-finding Nick, who has vanished, with no way to find him as she's on console. I, fortunately, never sent him anywhere whenever I had him, so he's forever in his agency. Dog, is the only half working companion I use. I say half, because the ammount of times I've left a building only to find dogmeat isn't at my side, while I haven't dismissed him... It's ridiculous!
Make an extra tab under Data. You have one for settlements, why not companions? Or are they un-trackable for some reason?
EDIT: Nick was sent to Greygarden. Summon bell doesn't work either. Giving him a bed hasn't helped... Nor even a chair