The Torah (Shmos 12:7, Devarim 16:4) uses the language of "your chametz shall not be seen". "Hebrew; Lecha".
The Gemara in Pesachim (5b) teaches that this means you may not see your own chametz on Pesach but you may see the chametz owned by others (example: a Gentile).
However, if you do sell the chametz to a Gentile before Pesach, The chametz needs a proper transfer act (kinyan) from the Jew to the Gentile. Also, the chametz should be removed from the Jew's property and delivered to the Gentile. A reason for this (besides kinyan) is to show that this is not a fake sale (since the goods were delivered to the Gentile's domain) and also because if the chametz is destroyed by accident, the Jew may be construed as being responsible for the lost chametz. The Gemara in Pesachim (5b; Rava's ruling to the people of Machuza) explains that we should remove even Gentile owned chametz from our homes if we are financially responsible for that chametz. (sources, see: Shulchan Aruch O.C. 448; Terumas HaDeshen 120; Magen Avraham 448:4)
In order to help relieve all of these issues (and since more and more Jews found themselves owning more chametz before Pesach) the Bach (Bach to S.A. O.C. 448:2) issued a ruling that the Jew could designate a space in his home (instead of the hard task of delivery) that he sells as real estate to the Gentile along with the chametz. Now the chametz has been "removed from the Jew's domain" , and "delivered to the Gentile".
However, the contract doing so need not grant the Gentile such exclusive right to the designated space as to automatically and completely bar the Jew's access to that space. It is enough that the Gentile own it or rent it from the Jew. The Gentile need not be consulted each time you wish to access the area designated for the Gentiles storage of delivered and sold chametz within your home. However, one should try not to access that space.
Finally, according to Shulchan Aruch (O.C. 440:2), you still should have the Gentile's area separated by a partition or covered over since we do not want Jewish household members to forget or be confused and access the chametz without thinking.
Therefore, since all of the above are precautionary measures, and not an absolute obligation to deny your access to the area, you can if need demands, access the area to retrieve an item you did not sell on Pesach.
I hope this helps. :)