First noticed this at the Fire Giants. We got to the third room or so before we headed back and encounters had started to respawn already.
Second case was in the Guardian Rest, with all the golems, where I clearly noticed this as the first encounter had respawned when we got out of the second room. We also ran into all of the encounters again after not having been there for very long.
Last case was in the cultist place in Cormanthor, I can't remember what it's called. Lost something. But we had barely finished the first level, and it didn't take much time, when we had to step outside for a moment and on the way back through it all encounters had respawned.
In one case we also ended up fighting two or three encounters at once as respawned encounters swarmed out of adjacent rooms because we didn't close the doors behind us but adventuring is just dangerous business, after all