If a person stole money or an object, can he return it without the owner’s knowledge?
If he must inform the owner, can he return it with an anonymous note? (ie, I regret that i stole this money and I am now returning it to you anonymously.)
If the owner did not know that the item was stolen, i.e. stole some candies from a basket and returned them, then you don’t have to inform the owner. If the owner did know that something was stolen, then it is sufficient to leave an anonymous letter. In this instance you would also have to ask him mechila. If the person knew that you stole from him, then you have to let him know that you returned the item to him, so he shouldn’t think that a someone else that stole from him returned the item.
CH;M 355-1, Igros Moshe CH:M 1 88 D:H Uma Sesha’alta Bos 15.