During Zuko's days back to the Fire Nation's capital city (and Iroh's imprisonment), Zuko finds a scroll telling him the location of a "Secret History" about his great grandfather's demise. Eventually we find out that it was Iroh the one who sent the scroll.
Iroh was imprisoned.
Of course he could break out anytime he wanted, but the time it takes to find a scroll, write on it, put it in front of Zuko's bedroom, and come back to the prison, is bound to be enough for anyone to notice that something's off.
So maybe someone else delivered the scroll on Iroh's command? I suppose that he had some allies (White Lotus members?), but I can't remember anything about that.
So the question is, how did Iroh manage to get that scroll in front of Zuko's bedroom while imprisoned?
Also consider that Iroh had a royal artefact hidden in a hole in the wall, which he later gives to Zuko. Where did he get it from?