I lost a companion once after telling them to wait somewhere then forgot where. Continued on with quests eventually got a message saying such and such got tired of waiting and returned to town. Went back to where I met them and found them there with the loot and stuff I gave them. Think it was 3 days in game maybe.
Because I've only just figured out that's where she could be! Will go check if she's still there after I've finished eating. If she's not, well, then I'm completely out of ideas. Do NPC's die of natural causes?
Why can't you just fast travel back to where you think you left her and tell her to then follow you?