I know of people who did a favor to a friend and loaned out their apartment. They knew the friend, but they did not know the people coming in to the apartment. How can they “trust” these people that they don’t know. They can only hope that they will treat their possessions with the respect that they obviously deserve. This friend of mine came home to find their house trashed. It was supposed to have been a couple and a small child coming to only sleep in their apartment. It turns out that there were 8 people there and they ate and had a big party there. What gives? These are frum people I would imagine.
Other cases I know of are people who are making a simcha and make an agreement with other neighbors in the area who are making a simcha but at a different week, so they agree to give their apartment to the other when they won’t be home. But in this case you can imagine that someone making a chassanah or Bar Mitzvah on Thursday has little time to make all the rooms spotless and presentable, or even throw out all the garbage and put all the laundry away before they dash off to their simcha out of town. They just throw everything into the master bedroom and lock it. Why do they have to clean it up just because someone is going to be staying in the other room/s of the house?
JF02, I think that sometimes people crack the door, see they made a mistake, and then become curious later on and go back to see personal effects. A person can feel invaded by this even if they planned on trusting the person.