In many situations, the monster/npc tactics in a dungeon go like this:
"If fight goes bad, one of npcs breaks off and alarms the evil fraction leader. Leader then assembles remaining forces and retreats to XYZ for last stand".
Sometimes, especially when my players run out of spells (or use spells harshly), they just simply go to sleep in a safe room or use spells like "rope trick" to get out of sight. The logic says, that the villain leader won't just wait in XYZ forever and probably will go to search for PCs or do some other intelligent decision. If this "sleep break" happens at the end of session, I do have time to adjust the tactics to this non-standard situation, but if it's in the middle of a session, I have quite a hard time to quickly get some kind of devious plan the villains do, while PCs are resting. And I don't like the idea "uh boys, smalltalk for 15 minutes, I have to think a little bit what next".
How do you guys solve such issues? Do you prepare in advance, improvise or do something else?