Objection 1. It would seem that it is not essential to theft to take another's thing secretly. For that which diminishes a sin, does not, apparently, belong to the essence of a sin. Now to sin secretly tends to diminish a sin, just as, on the contrary, it is written as indicating an aggravating circumstance of the sin of some (Isaiah 3:9): "They have proclaimed abroad their sin as Sodom, and they have not hid it." Therefore it is not essential to theft that it should consist in taking another's thing secretly.
Objection 2. Further, Ambrose says [Serm. lxiv, de temp., 2, Objection 3, Can. Sicut hi.]: and his words are embodied in the Decretals [Dist. xlvii]: "It is no less a crime to take from him that has, than to refuse to succor the needy when you can and are well off." Therefore just as theft consists in taking another's thing, so does it consist in keeping it back.
Objection 3. Further, a man may take by stealth from another, even that which is his own, for instance a thing that he has deposited with another, or that has been taken away from him unjustly. Therefore it is not essential to theft that it should consist in taking another's thing secretly.
On the contrary, Isidore says (Etym. x): "'Fur' [thief] is derived from 'furvus' and so from 'fuscus' [dark], because he takes advantage of the night."
I answer that, Three things combine together to constitute theft. The first belongs to theft as being contrary to justice, which gives to each one that which is his, so that it belongs to theft to take possession of what is another's. The second thing belongs to theft as distinct from those sins which are committed against the person, such as murder and adultery, and in this respect it belongs to theft to be about a thing possessed: for if a man takes what is another's not as a possession but as a part (for instance, if he amputates a limb), or as a person connected with him (for instance, if he carry off his daughter or his wife), it is not strictly speaking a case of theft. The third difference is that which completes the nature of theft, and consists in a thing being taken secretly: and in this respect it belongs properly to theft that it consists in "taking another's thing secretly."
Reply to Objection 1. Secrecy is sometimes a cause of sin, as when a man employs secrecy in order to commit a sin, for instance in fraud and guile. On this way it does not diminish sin, but constitutes a species of sin: and thus it is in theft. On another way secrecy is merely a circumstance of sin, and thus it diminishes sin, both because it is a sign of shame, and because it removes scandal.
Reply to Objection 2. To keep back what is due to another, inflicts the same kind of injury as taking a thing unjustly: wherefore an unjust detention is included in an unjust taking.
Reply to Objection 3. Nothing prevents that which belongs to one person simply, from belonging to another in some respect: thus a deposit belongs simply to the depositor, but with regard to its custody it is the depositary's, and the thing stolen is the thief's, not simply, but as regards its custody.
Source: Summa Theologica (New Advent)