Please always give us source and context so we don't have to go looking for it – your link might break someday. You may quote up to four sentences of text, as well as describing what is going on, such as what is the source: a book, a short story, old, modern, etc.?
From The Art of Deception.
"Ask Nicolas! He has neglected me because of that foolish painting." Lacroix took in the sight of his son, face and hands scorched, clutching a canvas to his chest. "So, what foolishness have you been up to?" "I have been painting", his child stated.
"What have you been up to?" is a very common way of asking someone what they've been doing. Said in a normal way, it's simply a polite question. But if you say it in a lilting, quizzical way, it suggests that you're pretty sure the person has been doing something they shouldn't.
That probably doesn't apply here, but the father does add "foolishness" to his question, following up on Janette's remark.