Signs your party is not the first in the Dungeon.

Sometimes players decide to keep returning to the same picked over part of the dungeon because it's safe rather then delve deeper.  This is especially common if it's a mega-dungeon.  best way of dealing with this is rival packs of murder hobos on a spree.  Treasure gets grabbed, enemies slain, allies turned hostile and a new extremely dangerous entry goes on the random monster table - the rival adventuring party.

Add this to your random encounter table...

Here's a table of  hints that the party might not be alone anymore.

D20	Signs that Rival Adventurers have Discovered this Location
1	A lockpick hanging loosely from a keyhole on the door.  50% door is trapped with a poison needle.
2	Graffiti on the wall, written with a torch stub (D4) 1- Obscene cartoon 2- Arrow or similar marker 3- Name of adventurer “Lovejoy Beast Throttler” 4- Symbol or motto of rival party.
3	A tortoise with a lit candle waxed to its back – crawling slowly about as a distraction.
4	Crude grave with broken sword headstone (or corpse covered in rubble) -  dead adventurer within has been thoroughly looted/mauled.
5	Walls, floor and ceiling are splashed with combustible oils
6	Unhappy pig tied firmly to the end of an 12 foot ladder.
7	Backpack containing: block of lard, iron filings, sack of flour, six iron spikes, mallet, lump of tar, flask of lye, 25 copper slugs painted gold, parchment and charcoal
8	Signs of desperate melee with blood and glob of flesh in surprising places
9	Room or hallway with furnishings burnt, blackened and incinerated
10	Sack with hand of fancy marble statute (12 GP)
11	Amateurish attempt at circle of protection written in chalk
12	Gruesomely mutilated corpse of dungeon inhabitant (or possibly fellow adventurer)
13	Dungeon vermin carcass spitted, cleaned, grilled and with one bite taken out of it.
14	All doors of this room are spiked from inside with long iron nails, room is empty.
15	Ten foot pole cut into crude stilts tied together with tarred rope.
16	Abandoned familiar seeking new master – will kill rival familiars. (D4) 1-Common Stoat 2- Large poisonous spider 3-Diposmaniac floating skull 4- Rat King (knows a level 1 spell)
17	A smashed picnic basket, checked cloth, silverware, scattered fancy potted delicacies and broken bottles of claret.
18	Sack of dead monster bits (roll random encounter for type of beast)
19	Message on wall, written in blood trailing off (D4) 1- “Balls!” 2-“Throg, I will always love yo…” 3-“L,L,L,R” 4- “The Yellow King Com…”
20	Broken chests, torn sacking and a single gold piece caught in a floor crack.