Dear Fr. NotSure,

Thanks for the question.

You said:
I am a priest and when hearing Confessions, I give absolution and say Go in peace. Then I make some small talk but then think of something that I should have said to them about the sins they just confessed.

• Is it permissible to bring up the sins that they just confessed?
  

My thinking is that Confession ends with Go in peace., and anything after that is outside of Confession.

The sacrament is over when you say a concluding remark such as Go in peace. If you think of something further you should refrain from saying it. The person is forgiven, even if the conversation was not absolutely complete, because the absolution was given in good faith.

After the Confession, it is not permissible to bring up the sins that had been confessed even in the small talk after.

From one brother priest to another, I hope this helps,

Fr. Jonathan