So, the nobility is expected to be very suspicious of the commoners known as adventurers. Presently, however, they only have the capacity to do very much good (fight the monsters and undead, sell unique items) and very little capacity to hurt the nobility (fail to repair and return unique items).
It has been proposed that the new estates would have some limited banking options. Sure, let's have it, but let's have it with some risk. Make it possible for adventurers to visit the estates of the region and attempt to steal gold and valuables from it. It would be up to the noble to pay for the guards, but it should also be relatively rare for the adventurer to be caught in the process, for understandably hard consequences would fall on him should the identity of the thief be discovered. In most cases the effort would fail "you did not find a good opportunity". Sometimes you would be discovered partially. "You have been spotted." The noble would receive a letter from his estate saying "A shady figure has been sneaking around your estate".
Further, when a theft succeeds, the noble is not informed immediately, but with a delay. Perhaps only when he visits the vault again.
Additional bonus would be that you would be less likely to be identified with lower prestige and higher distance from your home. This would create increased tension towards especially foreign outlaws. They would mostly be seen as an useful addition, but they would instantly have an air of suspicion about them.
Now IF we also had the possibility of leaving unique items to the vault and the possibility of having them stolen, this would generate even more possibilities. An item in a vault would degrade slightly slower than otherwise, but would then be exposed for theft. Items in vaults and high amounts of gold could also generate rumors that the adventurers could hear.
An adventurer rummaging through an estate could also come across some other information like random scribe notes or sometimes very rarely, letters written to the master of the estate. (if the adventurers were assumed to be able to read...).
But even with the ability to steal gold the adventurers would become quite more suspicious than what they are now.
Ps. They could perhaps also steal gold from temples and guild halls.
