Dogmeat spawned with me after 12 levels of absence.
How and why? I left him with Mama Murphy and assumed he stayed with her. Then I get an unfamiliar compass marker pointing at the elevator that leads to Mayor Idiot's office, and Dogmeat just shows up in the elevator next to me.
Golden Fool
If only he'd done that when I got him trapped inside Bethesda Comics from the Skyrim Power Armor mod, but no he refused to you his ability to teleport to join me outside