Where do npcs go once you remove them from their original cell? I used a command humanoid spell and moved a npc into a new cell. Is there anyway to determine where a specific npc like that has gone without using console commands? Surely they go somewhere?

I'm not completely sure, but after 72 in-game hours I'm going to assume that they return to their original spawn point (unless they are still in the same cell as you.)

If they don't appear there, then try the exterior cell of the Assarnatamat ruins. Since the ruins are located at Coordinates (0,0) some NPCs will end-up there by default.

It is my experience that they stay where you put them.
I remember that I once commanded fast travel NPC's and merchants to my Hlaalu stronghold.
I only did it once though, as from then on I had to recall to my stronghold if I wanted to use the Balmora and Hla Oad travel services.