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. In this way it does not diminish sin, but constitutes a
species of sin: and thus it is in theft. In 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 Obj. 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 Obj. 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.
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FOURTH ARTICLE [II-II, Q. 66, Art. 4]

Whether Theft and Robbery Are Sins of Different Species?

Objection 1: It would seem that theft and robbery are not sins of
different species. For theft and robbery differ as "secret" and
"manifest": because theft is taking something secretly, while robbery
is to take something violently and openly. Now in the other kinds of
sins, the secret and the manifest do not differ specifically.
Therefore theft and robbery are not different species of sin.

Obj. 2: Further, moral actions take their species from the end, as
stated above (I-II, Q. 1, A. 3; Q. 18, A. 6). Now theft and robbery
are directed to the same end, viz. the possession of another's
property. Therefore they do not differ specifically.

Obj. 3: Further, just as a thing is taken by force for the sake of
possession, so is a woman taken by force for pleasure: wherefore
Isidore says (Etym. x) that "he who commits a rape is called a
corrupter, and the victim of the rape is said to be corrupted." Now
it is a case of rape whether the woman be carried off publicly or
secretly. Therefore the thing appropriated is said to be taken by
force, whether it be done secretly or publicly. Therefore theft and
robbery do not differ.

_On the contrary,_ The Philosopher (Ethic. v, 2) distinguishes theft
from robbery, and states that theft is done in secret, but that
robbery is done openly.

_I answer that,_ Theft and robbery are vices contrary to justice, in
as much as one man does another an injustice. Now "no man suffers an
injustice willingly," as stated in _Ethic._ v, 9.