24 But one sometimes comes to a case of this kind, that we are not interrogated where the person is who is sought, nor forced to betray him, if he is hidden in such manner, that he cannot easily be found unless betrayed: but we are asked, whether he be in such a place or not. If we know him to be there, by holding our peace we betray him, or even by saying that we will in no wise tell whether he be there or not: for from this the questioner gathers that he is there, as, if he were not, nothing else would be answered by him who would not lie nor betray a man, but only, that he is not there.
So, by our either holding our peace, or saying such words, a man is betrayed, and he who seeks him has but to enter in, if he have the power, and find him: whereas he might have been turned aside from finding him by our telling a lie. Wherefore if you know not where he is, there is no cause for hiding the truth, but you must confess that you know not. But, if you know where he is, whether he be in the place which is named in the question or elsewhere; you must not say, when it is asked whether he be there or not, “I will not tell you what you ask,” but you must say, “I know where he is, but I will never show.”
For if, touching one place in particular you answer not and profess that you will not betray, it is just as if you should point to that same place with your finger: for a sure suspicion is thereby excited. But if at the first you confess that you know where he is, but will not tell, haply the inquisitor may be diverted from that place, and begin now to ply you that the place where he is may be betrayed. For which good faith and humanity whatever you shall bravely bear, is judged to be not only not culpable, but even laudable; save only these things which if a man suffer he is said to suffer not bravely, but immodestly and foully. For this is the last description of lie, concerning which we must treat more diligently.
Source: On Lying (New Advent)