Quote Originally Posted by Tamur
At times retreating units seem superhuman in their ability to move very large distances. I supposed it's possible that the game is suddenly generating odd bugs like this. More likely it's a case of units either retreating and merging, as professorspatula pointed out, or units retreating out of sight and staying out of sight, or simply wandering off and not showing up for years (thus causing the no Faction Destroyed messages). If it's possible in your version, using the "cheat" that removes fog-of-war is ideal for situations like this.
I can confirm that sometimes a beaten army get's a superhuman movement spurt if beaten in an enemy province.

I'm playing the ERE in vanilla RTW:BI, I had hired mercenary camel riders when trying to take Hatra from the Sassanids. After the battle was won I planned on disbanding them to avoid the cost, but as I had plenty of movement left I used them to ride into Sassanid territory to see what forces they had. Of course as soon as I got near the capital I was attacked by a full stack army, so I withdrew. I had run out of movement though so I couldn't withdraw when they attacked again, so I thought to myself I'll attack on the battle map and try and kill as many archers and skirmishers as I can. I did alright for a bit but of course the camels routed pretty quickly.
Here's the interesting bit:
I had completely run out of movement for the turn yet the 15 remaining camels suddenly retreated almost their entire movement allowance again to behind my border.Pretty much like the way armies move in MTW when defeated, retreating to the closest friendly border.