So a while ago I had Serana as a follower, and I was about to go into a cave, but I told her to wait outside. I couldn't find her for days until I came across her in a hallway at Fort Dawnguard. I had her follow me again, but at some point we got separated. Now I can't find her anymore.

Does anyone know where I can find her?

Try waiting for 1 hour.
I've often lost her when crossing mountains and rivers.
My only Dawnguard character usually treks far from established roads and losing followers is common.
But if I wait for 1 hour they tend to catch up. Give it a try.
Edit: If not, there's always the option of loading a previous Save.

Hope that helps

I'm afraid I don't have a saving to go back to, as this issue is quite old. Although I just miraculously found her again outside Fort Dawnguard's front door.

Is there a system where if someone goes X amount of time without being where they should be, they go back to their default location? (i.e., if a housecarl goes spends a week out in the middle of nowhere, she automatically goes back to your house). If so, where is Serana's default location?

DG has only been out since Tuesday, so the "issue" can't be that old. And, WTH, you're no stranger to this forum. You know you should be keeping manual saves, just in case.

And yes, followers are known to show up @ a standard home base after awhile. Based on your post, Serana's base is Fort DG.

I've now finished DG & Serana followed me without any problems over & through many a hill & dale. Or Vale. She's the most awesome follower yet. I hope they patch the voice command so it works better with her. She's a keeper.

Just give her time, she's been kept in a crypt for centuries.

(Also when I read the title, I pictured Captain Cutter's voice saying Serena. For Halo Wars players out there.)