DEUT1105 [lit. "until seek your brother"] - Now would it enter your mind to give it to him before he seeks it! How, then, is "until seek" to be understood? Seek him ["your brother"] out to determine whether or not he is deceiving you [in claiming the animal as his own]. And how [is he to be "sought out"]? By signs i.e., [by giving identifying signs, proving the animal is to be his] -- whence it is derived that [claiming a lost object by means of] signs is a Scriptural ordinance (Bava Metzia 28a)