But knowing confessed sins isn’t really reading a person’s mind.
OP, my understanding is that demons can be attached to certain individuals through their sin. By that I mean that a person who is clinging on to a certain sin, such as resentment or hatred, can very well be providing a type of grounding for demonic activity. It’s a sort of permission, and all mortal sin legally puts us under the authority of the evil spirits, who are very legalistic. Once a sin is confessed, however, it doesn’t have that sort of legally binding quality. Such a person is under the authority of Christ again.
I know that doesn’t really get at your question, but it might be helpful in some way. I don’t know enough to speculate further.
Fr. Amorth once told the story of a priest who was present during a possession, and who was hostile and who possibly interpreted the whole thing as mental illness or mere pretending. The possessed person manifesting a devil told him, “You don’t believe in me. But you believe in women. Oh boy, do you believe in women!”
So yes, sins can certainly come up, but I believe they usually come up as a way to undermine the efficacy of the exorcism.