I left Shalidor's Maze and hiked back to Whiterun. I didn't stop to notice that Stenvar wasn't following me until I got to the stables. I thought by passing through the town gates (loading screen) the usual workaround would work and he'd appear... but he didn't. So, I'm assuming some glitch had him stop following me near Labrynthian. Anyone else have this problem? I don't want to go all the way back there and look for him. Honestly, I wouldn't care but he's got some pretty good loot on him.

Did you try waiting? That may work if going into another area didn't. I can't tell you if it's a glitch because I've never had it happen to me, though I did lose Delphine and Esbern on the way to the temple, they just popped up when I got there.

Maybe this? (from UESP)

If Stenvar takes enough damage and is kneeling, he may not get up again and will be stuck in this position. If this happens, it is recommended to discharge him of his services and hire him again. Attacking or Shouting at him may also fix this.

I lost a follower recently and the otter day I found her wondering the wilderness. She wouldn't talk to me or let her be my follower. Sometimes followers just go missing and then appear following you again like nothing happened. Other times they spawn at their home and you have to get them to follow you again. But the rest of the time you have killed them by accident and they won't come back.