I found this exploit. Maybe was reported already.
It's simple: you can loot a house enterily and then reset the loot again trough the mission's of the trader.
Better explained: take a mission from the trader (for example "clear that house"), go to the POI and loot everything but not accept the mission. After you loot every corner go and accept officialy the mission. Once you accepted to clear the house, ALL the loot is restored again, EVEN THE ZOMBIES are restored again.
It happens in every single mission where there are POIS involved.