Here are some clear signs you've had adventurers come through an area.
All statues are destroyed. Or at least broken up and knocked over. After all, they could have been golems so they needed to be attacked. Then, if they weren't golems, clearly they were housing treasure or covering up some treasure.
Doors are destroyed, or spiked and locked and barred. Adventurers never forgive a stuck door, and slay them utterly. On the other hand, a door that will open clearly can be used to get the adventurers from behind. Those will be locked, barred, jammed, and spiked to the best of the adventurer's abilities. If those doors lead nowhere, so much the better. Clearly there is a secret door they just can't find past it which conceals grave danger.
Traps are re-set. Pretty much, if a trap can be re-set after what it guards is gone, delvers will have done that. A group of thorough and expert adventurers will leave behind a heavily trapped corridor to a locked (and spiked!) vault door to an empty vault. They may add their own traps, just to get future adventurers who might try to loot their already looted places. Great is the fear of, and jealousy of, other adventuring groups.
Furniture is hacked up. Or, honestly, just gone without a trace to be sold to "collectors" the delvers have convinced themselves are out there. You know, the kind that buy moldy old dungeon furniture at a markup. The rest will have been hacked into tiny pieces to ensure there wasn't any treasure hidden inside, legs broken off to see if they're actually staves, wands, or rods, and chair cushions carefully sliced open to check for gems. Mirrors will be broken, in case they spring magical evil twins.
Bodies are dismembered. The intact ones have already walked off as undead servants under the control of the party's spellcasters. The others have been thoroughly rendered lootless. Teeth, possibly valuable organs, skins, clothing, weapons, fingers that could conceal loot, etc. have been cut off.*
Weapons are broken. No adventurer is foolish enough to leave weapons lying around. If they can't be taken immediately to be cashed in in town, they'll be cached secretly for later. If that's not practical (or simply not trivially easy), they'll be destroyed. Hacked up, smashed, burned to cinders, and otherwise ruined. If that's not possible, they'll be trapped and/or hidden.
Dead ends are heavily marked up. A dead end always has a secret door, therefore all dead ends will be scorched, hacked, marked with pick marks, and tapped. They may also be trapped, to nail any rival adventurers attempting to find the as-yet-undiscovered secret door the original delvers couldn't find in their previous half-dozen attempts to find the door that must be there.
More to come in the future I'm sure . . .
* By the way, in any game system, this only takes "one round." That's 1 second, 6 seconds, one minute, etc. - plenty of time for a complete and thorough search that finds all loot. Usually this can be done simultaneously with other actions, such as fighting, running away full speed, casting spells, or looting other bodies.