By the Bais Hora'ah
I observed a friend secretly hiding a recorder to record someone’s private conversation.
Q: Obviously, it is improper to secretly record a conversation, but does doing so violate an actual prohibition? Would it be considered geneivas daas?
A: It is evident from your question that you are translating geneivas daas literally: to steal someone’s thoughts. However, in halachic literature (C.M. 228) it is classically described as the act of misleading and deceiving someone in a manner that will cause this person to mistakenly feel morally indebted, even though he is not actually indebted. Thus secretly recording someone does not violate the prohibition of geneivas daas. Nevertheless we do find authorities who would consider such an act geneivas daas (see Chikekei Lev 1, Y.D. 49 and Pele Yo’etz, Geneivah).
But there are additional issues to consider. This act may possibly violate the Cherem D’Rabbeinu Gershom (Be’er Hagolah, Y.D. 334), who prohibits reading other people’s correspondence. It is debatable whether this ban extends to all manner of communication or whether it is limited to written correspondence. It is obvious that, at the very least, it constitutes a violation of v’ahavta l’rei’acha kamocha (see Chikekei Lev ibid., which suggests this as the underlying rationale behind Rabbeinu Gershom’s cherem).
Additionally, some write that it is prohibited to reveal a friend’s private concerns, which is an extension of the prohibition of tale-bearing (rechilus). If one may not tell tales to others, one may certainly not seek tales for himself (Halachos Ketanos 1:6).
Some contend that secretly recording conversations is a form of hezek re’iyah — the prohibition against causing damage by gazing at another. It is broadly defined as the prohibition against violating another person’s privacy, even if he is aware of it, as he might be too embarrassed to protest.
Chazal relate that when Bilam observed that placement of the tents of the Jews was done in a way that assured each one’s privacy, he commented that that made them worthy to receive the Divine presence (see Shulchan Aruch Harav, Nizkei Mammon 11:13). The obvious extension of this principle is that secretly recording a conversation is a violation of privacy and represents a lack of tznius.
Nevertheless, for purposes of chinuch it may be permitted to secretly record a conversation when necessary (see Rashba 1:557), for example, to afford one the opportunity to prevent someone from sinning or to be able to recover money that is owed to him (Pele Yo’etz).