Random table: While the adventurers are away...

Every month or week (depending on how active the monsters are) the monsters heal and/or reset their traps.
In addition, roll a d12, or multiple d12s if the monsters are particularly threatening.

D12 - RESULT
1 Nothing.
2 Monsters roam the countryside, raid farms, capture people. Nearest village's economy affected, local prices go up, some things unavailable.
3 Monsters raid or destroy nearest village significantly.
4 Monsters improve their war assets, roll 1d4: create new traps, build a siege engine or similar, get better equipment, use better tactics.
5 Monsters succeed on magical research, roll 1d4: create magic traps, summon guardians (e.g. demons), learn spells, learn alchemy or similar.
6 Monsters gain a tough leader (use a new monster or upgrade an existing one) with a specialty from the war assets or magical research above
7 Monsters gain reinforcements.
8 Monster faction allies with another faction from inside or outside the dungeon. Turning them against each other will be difficult.
9 Monsters establish a foothold outside the dungeon, become more of a threat.
10 Monsters move on to a better, badder dungeon, take all the treasure with them.
11 A faction (roll for faction) or adventuring party (roll for alignment) delve into the dungeon. Roll 1d6 for result below.
12 Roll two more times.

Faction delve results:
D6 - RESULT
1. Total failure (enslaved, ransomed, killed; the monsters take their stuff, 50% chance of a lone survivor)
2. Desperate retreat (as above with more survivors)
3-4. Middling results (remove 2d6 unexplored rooms of monsters and treasure from the dungeon)
5. Decent success (4d6 unexplored rooms)
6. Triumphant return (6d6 unexplored rooms)

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EXAMPLE (for my own reference; using a standard necromancer lair and filling out the details.)

Week 1
9 - FOOTHOLD: The necromancer's apprentice now lairs in the deserted village of Oldberry.

Week 2
10 - MOVED: Necromancer abandons his lair for the ruined fortress of Kara-ang.

Week 3
9, 4 - FOOTHOLD, WAR ASSETS (TACTICS): The apprentice builds a tower outside the village. Meanwhile, the necromancer improves its undead's intelligence and battle tactics.

Week 4
5 - MAGICAL RESEARCH (SUMMON): The necromancer reanimates a giant's skeleton to act as a guardian.

Week 5
10, 11, 8  MOVED, FACTION DELVE, ALLIANCE - The apprentice moves back into the necromancer's old lair. Following these news, the Lord's Men venture into the lair and... (6) clear it out entirely (17 rooms), slaying the apprentice! During that time, the Necromancer allies with the bandits that lair around the ruined fortress.

Week 6
5 - MAGICAL RESEARCH (TRAPS): The necromancer creates a terrible field of magical wilting around his domain.

Week 7
4 - WAR ASSETS (ENGINE): The skeletons atop the ruined towers of Kara-ang now man ballistas.