Where to return a lost object

If hanging on the door or in their mailbox is a safe place, meaning no one would take it then it would be permitted to hang it on their door. For example they would be very comfortable leaving stuff oof their own there without worry of someone taking it. However if you are unsure of the safety if you put it there at the time when they are home and going to open the door soon to leave the house, get the paper etc.. you do not have to notify them before returning it.
The Gemara in Baba Kama 57 a discusses what level for responsibility one has when returning lost objects. It says: If one found a lost item and he returned it to a place where its owner will see it, he is no longer responsible to deal with it. If the item was then stolen or lost, the finder must compensate for the loss…
this is what it is teaching: If the finder returned it in the morning, when people are typically present, to a place where the owner of the lost item will see it, and the owner typically enters and exits and it is likely that he will see it, the finder is no longer responsible to deal with it, nor does he bear responsibility if it is then stolen or lost. By contrast, if the finder returned it at midday to a place where the owner will see it, since it is a time when the owner does not typically enter and exit and will not see it, if it is then stolen or lost the finder bears financial responsibility for the loss.
From the word “hashev” I have derived only that one may return the item to the house of the owner of the lost article. From where do I derive that even if one returns it to his garden or to his ruin, i.e., an unused structure on his property, he has discharged his obligation and is no longer responsible for the item that he found? For this, the verse states: “Teshivem,” repeating the verb for emphasis, to teach that he fulfills the mitzva by returning the item to any place belonging to the owner.
What is the meaning of the phrase in the baraita: To his garden or to his ruin? If we say that the finder returned the lost item to the garden of the owner that is secured, i.e., properly enclosed, or to his ruin that is secured, it would be unnecessary to state this, as it is the same as his house, since these spaces are secured in the same way as his house. Rather, it is obvious that it means that he returned them to his garden that is not secured, or to his ruin that is not secured, and nevertheless, the finder is no longer liable for subsequent damage or theft of the found item.
Rabbi Elazar says: All those who are obligated to return items to their owners, require the owner’s knowledge that they are returning it except for one fulfilling the mitzva of returning a lost item. This is because the Torah included many permitted ways of returning lost items by employing the double expression “hashev teshivem,” which serves to permit the return of the lost item without the knowledge of the owner.
The Shulchan Aruch Choshen Mishpat 257:1 rules: One must care for a lost object until it returns to the property of its owners in a safe place. However if one returns it to a place which is not safe like a garden or empty area and it gets lost there he is required to pay for it. If h returned in the morning to where the owners enter in and out of their house in the morning you do not have to care for it anymore because they will see it, even if the place is not a safe place per se… And you do not need the inform he owners.
So If you put it by their door in the morning when they are going out anyway then that is fine. If not then if you are sure that area is safe you may leave it there.