Do zombies repopulate in areas you've already cleared out?

After playing three different levels and testing this I can confirm that idle zombie do not repopulate parts of the level you have cleared out.

I tested this on these Left 4 Dead 2 levels:

• The Parish - The Quarter

• Death Toll - The Turnpike

• Crash Course - The Alleys

Crash Course's The Alleys is notable for being an especially long level yet while backtracking I only encountered a one horde event. In fact, after testing I think special zombies do not respawn in areas you've cleared out either. I did not encounter hunters, chargers, boomers, or the like during my backtracking. I did re-encounter a witch I had previously navigated around which indicates that they do not despawn even when left alone.

While I performed my testing on Normal difficulty I think that this information will be especially useful for players playing on Expert. On one occasion I was able to modify my weapon with a laser sight but quickly had to replace after I ran out of ammunition. I knew I could safely return to areas I'd traversed and I was able to go back and upgrade my new weapon from the laser sight container I'd passed earlier.

Lastly, I'd like to point out that I occasionally had to fight an incoming horde of zombies regardless of where I was. Meaning that idling common infected stop spawning when backtracking, but horde events can still occur.

The only question I still want to test is what's the respawning behavior like on The Parish - Bridge finale gauntlet event. I still want to know if the zombies do not stop spawning while on the bridge and if I could return to the safe room if I truly needed to, in the same way I could during the previous levels.