So I let go of a companion, and the message immediately appeared saying that he was lost (I assume he disappeared from the game instead of going to nearby tavern?). Despite not having him in my party anymore, he still shows up in battles with his default gear. I can take him to hideouts, I can assign him to formations, he participates in battles.
Here's the save file, his name is Eronys the Scholar:
Here's the save file, his name is Eronys the Scholar: