Yeah, apparently this game is as bad about keeping companions easily accessible as the last two were. Normally you should be able to send them to a settlement and keep them there, but there's no preset place in the settlement that you'll find them at (ex: if there's more than one doghouse, like in Sanctuary, then Dogmeat wanders between them), some will default to where you originally met them (ex: Curie keeps going back to Vault 81), and some are bound to a particular spot if there's an active question that involves them (Piper/Nick at their offices).

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CdrMike, Overwatch Reject