So there I am exploring the Soul Cairn, trying to find Arvak's pretty little head, and I notice that Serana is nowhere to be found. Search high, low, try waiting an hour, 2 etc. Negative contact. Not at the quest marker waiting. Retraced my steps. Notta. She was there, then not. This a common bug, or is this a weird random occurence?

Companions tend to wander off there chasing the angry ghosts.
J'zaro exhibited it as well as Serena when I had a multiple companion mod installed.

Best way to retrieve a wayward companion is to enter any area that requires a loading screen.

There is a place with its own cell in the Soul Cairn that you can enter. Maybe that'll fix the problem.

Yeah, I love Serana... but I freakin' hate her too! I'm always turning around thinking, "Where the heck are you now??"

I'd go for a new cell within the Soul Cairn as fupark says. She gets all confused especially when there are bonemen or wraithwhatevers on top of towers. I had her stuck on the ground once. She had her spells out and ready to fire but wouldn't move at all. I couldn't speak to her, push her.. nothing. She just sat there. I bet she would have stayed there the whole time. I ended up going on top of the tower and killed whatever it was up top and suddenly she says, "Done is done." Then she sheathes her spells and starts walking to me.

Every world map in a game has 'nodes', that are a kind of script that tells NPCs how to move around and which routes take. You, the human player, don't have to follow these nodes, because, of course, you have a brain that tells you where to go. So when you move around the map in a messy way, (jump off of buildings, jump through obstacles, etc) NPCs don't, and, instead, will reach you by following the nodes. Usually, the more you move in a 'messy' way, the more NPCs get dumbed down because they have to follow a numerous amount of nodes to reach you.

It Always happend to me when I weren't just walking without jumping or skipping buildings, and infact Serana had a hard time following me, but eventually managed to get to me all the times. The more you don't follow 'regular' paths, the more your follower/s will reach you more with difficulties.

Entering another cell should teleport her to you.