I think that Parris' lack of answer on this question is what ends up feeding his own insecurity. The fact that he cannot formulate a full explanation as to where Abigail and Mercy have gone helps to erode his once strong and confident foundation. He hears some small rumor of the fact that the girls might have gone aboard a sea vessel from his daughter, but that's really all he has. He has little idea as to where both girls are and this strikes at his own fear and doubt. He based everything he did in terms of rallying the town regarding the fear of witches based on Abigail's persuasion and her own testimony. As she is gone, Parris understands that a major element of both the case for witches in Salem and his own credibility has gone with it and in this, there is a sense of fear. It is for this reason that he really lacks a clear answer as to where both Abigail and Mercy have gone. All he knows is that their disappearance does not spell good things for his own case and his standing in Salem. Given the girls' disappearance and the rebellions in Andover, Parris knows that his own time in Salem is coming to an end, the only answer he can ascertain at this point in the drama.