I had cleared out (= killed everyone in) White River Watch the previous game day. Just now, as I was returning from Valtheim Towers, the random encounter area between the Ritual Stone and WRW spawned one of those bandit groups with two Chiefs.
Well, they didn't insta-aggro, so I decided to follow them to see whether they would try to cross the bridge near Honningbrew (because I was not going to allow that). They headed up the path towards WRW, and the Chiefs actually entered. The two lesser bandits turned hostile before they could follow their leaders.
After killing the Bandits, I followed the Chiefs again. Not particularly smart, as I was level 6 with fur armor... but it was the first time anything like that had ever happened to me; bandits entering a cleared dungeon.
Inside, I immediately ran into two Bandits, one of whom was a mage (there were no mages there when I cleared WRW). Killed them easily enough, and continued deeper to find the male Chief casually leaning against the wall. His female friend was nowhere to be seen. Aggroed him (and he very nearly managed to kill me - luckily I had a level up), and after winning continued on.
All of the bandit corpses I had produced on my first visit were stripped of their entire inventories, even though I had only taken some arrows and misc items from them...
WRW has a 30-day reset time, so why did those bandits suddenly spawn there after ONE day? And where did the female Chief go? She wasn't inside, however I saw her enter.
So many mysteries...
Maybe Skyrim just felt like trolling me.