The Catholic Encyclopedia gives an example of “wide mental reservation” which it states does not constitute a “lie”. The example is responding to a question as to whether somebody is “at home”. So, for example…

… I go to Damascus’ house and ask his butler “is Damascus home?” The butler, knowing that I am a disagreeable fellow and that Damascus does not wish to talk to me, uses the device of “mental reservation” and says to me: “Sorry sir, Damascus is not home”. According to the Catholic Encyclopedia, this would not be a “lie” if the butler adds silently in his mind “for you”. According to the Church, this would be acceptable and would not constitute a “lie”. (Now, the example is not a grave matter, but it
is the example used by the Encyclopedia).
Now, to my mind, the butler is a
liar because he knows very well that Damascus is home and, yet, says directly to me, that he “is not home.” This is telling a falsehood ("he is not home when, in fact, he is certainly home) with the intent of deceiving the the person to whom the falsehood is told.
Catholic Encyclopedia Excerpt:
“Such expressions as “He is not at home” were called equivocations, or amphibologies, and when there was good reason for using them their lawfulness was admitted by all. If the person inquired for was really at home, but did not wish to see the visitor, the meaning of the phrase “He is not at home” was restricted by the mind of the speaker to this sense, “He is not at home for you, or to see you.” Hence equivocations and amphibologies came to be called mental restrictions or reservations. It was commonly admitted that an equivocal expression need not necessarily be used when the words of the speaker receive a special meaning from the circumstances in which he is placed, or from the position which he holds. Thus, if a confessor is asked about sins made known to him in confession, he should answer “I do not know,” and such words as those when used by a priest mean “I do not know apart from confession,” or “I do not know as man,” or “I have no knowledge of the matter which I can communicate.” All Catholic writers were, and are, agreed that when there is good reason, such expressions as the above may be made use of, and that they are not lies.”