The scenario was clearly somewhat of an ethical dilemma that Avraham had to balance. The Yalkut Shimoni, Parshas Vayera 98 notes:
באותה שעה הרהר בלבו אברהם אם אני מודיע לשרה נשים דעתן קלה ואם לא אודיעה ואגנבנו כיון שאינה רואה אותו תחנק את עצמה
At that time, Avraham thought to himself, "If I make known to Sarah (G-d's command to sacrifice Yitzchak) - women are light-headed. But if I don't make it known, and I steal him away, when she sees that he is not there she will kill herself (out of anguish)."
Thus, the Yalkut continues that he requested that Sarah prepare food and drink so that they could rejoice. When she asked what exactly this 'rejoicing' was for, Avraham responded that they merited to have a son in their old age. He goes on to say that he first discovered Hashem aged three, now that Yitzchak was an adult he needed to be educated, and as such, he would bring him to a place to be properly trained. On hearing this Sarah gave him her blessing.
Avraham consequently rose early the next day to ensure there was no chance for Sarah to change her mind and that they would not be seen by others. It was at this juncture that the Satan arrived. Firstly, he appeared in the appearance of an old man and asked Avraham where they were going. Avraham answered they were going to daven, to which the Satan queried, why he then also needed wood and the knife etc. Avraham promptly responds in case they get delayed and need to stop on the way. The Satan then went to Sarah and asked where Avraham was. Sarah answered that he was at his work. He then asked about Yitzchak's whereabouts, which she said that he was with Avraham. The Satan then interjected, 'But didn't you say that you wouldn't let him out of the house?' Sarah replies that he has not gone to work, but rather to pray. The Satan concludes the conversation by saying that you will not see him again, to which Sarah answers 'May Hashem do as He wills with my son.'
So it would seem that Avraham had to grapple with this moral dilemma. On the one hand he didn't want to say it outright, but by the same token, if he spirited Yitzchak away it would also have ramifications. Therefore, perhaps to a certain degree whilst he didn't tell her completely, he subliminally prepared her. To the point, whereby when the Satan said that she will never see Yitzchak again, she answered with absolute faith and conviction that Hashem will do as He sees fit.