I think the dnd rules are a little vague on this, perhaps intentionally. It does clearly state you fail a pickpocket if the target passes a perception check, but that doesnt mean every undetected attempt passes nor that you cannot steal when someone notices.
Had a player once try to pickpocket another, it didn't go well.
They passed the perception check, and also rolled a successful hit. Dude dropped like a sack of potatoes.
