May one enter another person’s property to retrieve an item that fell inside, such as a ball?
It is forbidden to enter another person’s property to retrieve a lost object, such as a ball, without their permission, if the property is fenced or gated, or one knows that trespassing is forbidden. If the property is open, such as the front lawn or the driveway in front of a home, it is permitted to do so. Nonetheless, in all cases, the property owner is obligated to return the lost object to its rightful owner, and withholding it transgresses the command of Hashavas Aveida and Biblically transgresses the prohibition against stealing. [This applies even according to state law, in many countries. It is illegal to withhold someone else’s object, including a lost item that was found, and one who does so can be charged with larceny, or theft by conversion. This applies even if one is annoyed at how often the ball of his neighbor falls into his yard, nonetheless, according to Halacha and the law of the land, he must return it and can face legal penalties if he does not do so.]
What is one to do if the owner cannot be found or does not want to return the object? In all cases that one is unable to receive the item back from the owner of the property [such as if the owner is out of town, or refuses to return it], he is allowed to take matters into his own hands, and enter the property to retrieve his item. [This may apply even according to state law. It is advisable to discuss the matter first with an attorney.]
 The prohibition to enter another’s property: There are several sources which back this prohibition: 1) So rule regarding a stolen home and seemingly the same would apply to using someone’s property without their permission: Admur Hilchos Gezeila 11 “It is forbidden to enter the home for shade from the sun, or protection from the rain, or to pass through the field”; Michaber C.M. 369:2; Rambam Gezeila 5:3; 2) So rule regarding the rule of Zeh Nehneh Vizeh Lo Chaser that we never force another to let someone else in their property: Rama 363:6 3) So rule regarding stealing with intent to return: Admur Gzeila Ugineiva 3; Michaber C.M. 348/1; Rambam Hilchos Gneiva 1/2; Learned from Braisa Bava Metzia 61b “Liminkat” regarding stealing in order to pay Keifel [See Kuntrus Achron ibid 1] 4) By a private home or area, this is also forbidden due to Hezek Reiya: Admur Nizkei Mamon 11-13; Rama 154/3; Michaber 357/1 “Hezek Reiyah”; Tur 157; Rosh Baba Basra 1; Bava Basra 2b
The prohibition to enter another’s property to retrieve an item: Entering another’s property in order to retrieve an item falls under the law of “Adam Oseh Din Latzmo-Taking justice into his own hands” which is permitted to be done, since one is taking back an item that he knows belongs to him. [Admur Hilchos Gezeila Ugeneiva 28; Michaber 4:1; Bava Kama 27b] However, initially we rule that one may not enter the other persons property to retrieve the item, as it appears that he is stealing from him. [Admur Hilchos Gezeila Ugeneiva 28; Ben Bag Bag Bava Kama 27b] Furthermore, in this case one can say that it does not just look like one is stealing, but one is actually stealing, as the property owner did not have negligence involved in the fact the person’s item fell on his property. Thus, it makes no sense to say that his property and privacy can be invaded without his permission for someone to retrieve an item, when he has done nothing wrong, and on the contrary, it’s the negligence of the owner of the item, who allowed it to fall in his yard. Accordingly, while I have not found any direct source in Poskim for this case, it seems it would be considered actual stealing to enter the property without permission, and so is Lehavdil the state law.
The reason: Entering another’s property without permission is prohibited due to stealing and if it is a private area, such as a private home, it is also forbidden due to Hezek Reiya. Vetzaruch Iyun if the prohibition is a Biblical transgression against stealing or not.
Other opinions: Some Poskim rule it is only forbidden to steal with intent to keep the item, however to steal temporarily is permitted, and so is done on a daily occurrence. [second Pirush, and conclusion, in Shita Mekubetzes on Gemara ibid, brought in Pischeiy Teshuvah 348/2 and Ketzos Hachoshen 348/1; See Smeh ibid who seems to learn in Michaber that stealing for pain is to keep the item; Pischeiy Teshuvah 348/2 and Ketzos Hachoshen 348/1 conclude it is proper to be stringent] Seemingly, according to this opinion, the same would apply to entering someone’s property without intent to steal. However, in truth, there are other prohibitions involved here, such as Hezek Reiyah, which certainly would apply even if one only enters temporarily.
 The reason: As in such a case the owner does not mind people walking on or through the area, such as when a person comes to knock on the door, and it thus not viewed as trespassing at all, to enter this area. Certainly then, one may retrieve an item from the front lawn or driveway without asking permission of the owner.
 Admur Hilchos Metzia Upikadon 1 and Michaber C.M. 259:1, and Rava Bava Metzia 26b that one who does not return a lost object transgresses the negative command of “Lo Sialeim” and also the negative command Lo Sigzol”; and nullifies the positive command of “Hashavas Aveida”
 Admur Hilchos Gezeila Ugeneiva 28; Bava Kama 27b and Tosafus there that even according to Ben Bag Bag, if there is a loss one may enter the property
 In such a case, the allowance to enter the property is not exactly clear from Admur ibid, as in that case it discusses a landowner who stole the person’s item, and hence one can say that he forfeits his right against trespass if I come to retrieve my item. However, here, that the owner does not even know the item is in his property, perhaps the owner of the item would need to wait to get permission to enter, as the landowner did nothing wrong of his own. However, in truth, this is incorrect, as the entire idea of Adam Oseh Doin Leatzmo is that he takes the place of a judge and Beis Din [See Sheim Derech Bava Kama 58], and a Beis Din has ability to retrieve an item from another, even if the other person does not know of it. Vetzaruch Iyun
 In such a case [i.e. withholding a lost item], the landlord is considered to be stealing [Admur Hilchos Metzia Upikadon 1] and is the exact case mentioned in Admur Hilchos Gezeila Ugeneiva 28 which permits one to enter the property to take his item if he cannot retrieve it otherwise.
 The reason: As this is a typical case of “Adam Oseh Din Leatzmo”
 See the following articles regarding State law: http://www.dummies.com/education/law/property-rights-what-constitutes-a-trespass/ ; https://www.hg.org/article.asp?id=37066;https://www.avvo.com/legal-answers/my-personal-property-is-on-someones-property-and-t-686190.html ; https://en.wikipedia.org/wiki/Repossession; I was advised by a lawyer in Israel who specializes in Israeli property laws, that if one exhausted all options of retrieving his item, such as he knocked on the door went to neighbors, tried to call the property owner, and was still able to reach him, then he may enter the yard to retrieve an item that accidently fell inside. It is however a good idea to take a video of one’s attempts to reach the owner, in order to defend himself from any legal suits that he may face for trespassing.