So I completed a dungeon, ran out and didnt see my follower.

I went to my breeze home and waited 24 hours... nothing...
I slept another 10... nothing.
Spawned to another location... nope
and looked in the castle (where I found her one time) and again nothing.

I am on the PC so luckily I used the "~" command to warp to my companion.
Ofcourse she was in the last dungeon, so I probably could have found her in less than 30 minutes.

But if I would have not noticed sooner, say a few different locations, and forget exactly where I have been... she would have been lost/stuck forever?

You were too impatient. It takes 72 game hours before the follower gets bored and goes back home. Then it will take a game day or two for them to get there.

And, yes, I have had a follower get thrown into the void in a dungeon, it happens. Well, it only happened once to me. I had to reload to may save point from before I went into that dungeon.