Bava Batra 39 - How to Lodge an Effective Protest and Confidentiality Agreements
If the original owner merely tells two witnesses, "So-and-so is a thief," this is not an effective protest. If, however, he says, "So-and-so is a thief, for he has taken possession of my land through an act of theft and tomorrow I shall claim it from him in court," this is an effective protest. The threat to sue is not really necessary, but this is how people usually conclude their protest.
If the protester tells the witnesses, "Don't tell the occupant that I lodged a protest," we assume that he means not to tell him directly, but they may tell others. Thus, this protest is effective. If the witnesses promise on their own, "Not a word of protest will leave our mouths," there is still a chance that they will reveal it inadvertently. Only if they are told, "Keep this completely confidential," can they be relied upon to keep it secret.