The stories I've heard where Signal messages have been extracted or otherwise accessed was from beyond either end. Someone invited a journalist to a private group chat. Someone handed someone else an unlocked device. The most alarming one is apparently Apple uploads every push notification your device gets to their servers. So if you are concerned about privacy there's a feature in Signal to set push notifications to only say "you got a message" and not include the sender or message contents in the notification.
I haven't heard of Signal itself leaking messages.