In Pesachim 63b R. Yochanan says that one is Chayav for Shochet Al ha'Chametz even if the Chametz is not in the Azarah, since 'Al' isn't Samuch. There seems no reason to put another limit on where the Chametz must be - even in China!
But as R. Akiva Eiger notes (Gilyon ha'Shas here), the Rash in Kelim 1:8 seems to understand that just as Lachmei Todah can be out of the Azarah but must still be in Yerushalayim, so too the Chametz has to be in Yerushalayim. That seems very strange - where could the Rash have taken that from? The Lachmei Todah are Pasul out of Yerushalayim, so even if they don't have to be 'Samuch', they must be in Yerushalayim. But why must the Chametz be in Yerushalayim in order to transgress this Lav?
Did you see anyone who discusses this?
1) The Gaon Rav Yehoshua of Kutno writes in Yeshuot Malko (Kiryat Arba) on the Rambam Hilchot Korban Pesach 1:5 DH Hashochet, that the Gemara here 63b cites the verse (Shemot 34:25) "You shall not slaughter the blood of my sacrifice on Chametz, and you shall not leave over till the morning the sacrifice of the festival of Pesach". The Gemara states that we learn from here that one only transgresses the prohibition of slaughtering on Chametz, if this is a korban for which there is a prohibition against leaving overnight.
2) Yeshuot Malko writes that it is possible that it follows that a similar Halacha applies also concerning the place of the korban. One only transgresses the prohibition of slaughtering korban Pesach on chametz, if the Chametz is located in a place where one could transgress the prohibition of leaving the korban overnight. This means exclusively Yerushalayim.
3) This is because in Yerushalayim there is a postive Mitzvah to eat the meat of the Pesach, and a negative Mitzvah against leaving the korban overnight. This does not apply outside of Yerushalayim, because when the korban leaves Yerushalayim, it becomes invalidated because of "Yotzei". If the korban became pasul because of Yotzei one cannot transgress with it an additional prohibition of "Lo Yalin"; leaving overnight.
I found, bs'd, other Mefarshim who explain on similar lines to that of the Yeshuot Malko.
(a) Chidushei Maharitz Dushinski here DH uLeDivrei, also writes that the reason that Rash Keilim 1:8 maintains that the Chametz must be in Yerushalayim even according to Rabbi Yochanan, is not because one requires ?? ????? but because if it is outside Yerushalayim one does not transgress Lo Yalin since it becomes Pasul by leaving Yerushalayim.
(b) The Maharitz Dushinski DH veNireh offers a proof for the Rash in Keilim from our sugya. This is from the question of the Gemara; why do Rabbi Yochanan and Resh Lakish need to have their dispute twice; both with Chametz inside/outside the Azarah at the time of slaughtering the Pesach and also about the bread of the Todah inside/outside the walls of the Azarah?! Maharitz Dushinski writes that if it would be true that according to R. Yochanan the chametz could even be at the other side of the world, then it would indeed be necessary for the dispute between R. Yochanan and Resh Lakish to be stated twice; to stress that Chametz is not similar to Todah, and according to R. Yochanan the Chametz need not be inside the walls of Yerushalayim but could be anywhere in the world.
2) However, the sefer Chukat Hapesach (Gershuni) Hilchot Korban Pesach 1:5, end of page 28 DH uMah She-Tafas, writes that the Tzalach, by the Noda biYehuda, does not agree with the Yeshuot Malko. The Tzalach is on Pesachim 28a DH veLachein Ani Omer. He writes that he has a chidush that he has not seen written by any of the Rishonim. This is that the warning against Notar; that one may not leave the korban over till the morning; applies whether the korban is kasher or pasul. He cites a proof for this from Pesachim 70a, where is a verse that teaches that the prohibition of breaking the bone of the korban Pessach only applies for a kosher Korban, not for an invalid one. Tzalach writes that if not for this verse, one would transgress the lav even with an invalid korban. It therefore follows, that for Notar, where there is no special verse, one transgresses even if the korban was invalid because it had left Yerushalayim.