The Shulhan Aruch (Orah Haim 178:1) rules that if a person was eating a meal with bread in his home, and in the middle he leaves and goes to another house, and no one is left behind at his table, this constitutes a "Hefsek" (interruption) in the meal. As such, when he returns he must recite Birkat Ha’mazon for what he already ate – assuming he ate at least a Ke’zayit of bread – and then recite a new Beracha of "Ha’mosi" before he resumes eating. This is the opinion accepted as normative Halacha by Hacham Bension Abba Shaul (Israel, 1923-1998), in his Or Le’siyon (1:17, and elsewhere), as well as Rabbi Moshe Halevi (Israel, 1961-2001), in his Birkat Hashem.
However, the Rama (Rabbi Moshe Isserles of Cracow, 1525-1572), reflecting the view of the Ashkenazim, disputes this ruling, and maintains that since the person was eating bread, he is still connected to his original location even after he leaves. As such, he may continue eating when he returns, without having to recite Birkat Ha’mazon or repeat "Ha’mosi." This applies even if he did not leave anybody behind at the meal, and he was eating alone. The Rama’s ruling is based on the position of Rav Hisda mentioned in the Gemara.
In light of this difference of opinion, Hacham Ovadia Yosef applies to this question the famous rule of "Safek Berachot Le’hakel" – that one does not recite a Beracha in situations of uncertainty. Accordingly, he rules that one should not recite Birkat Ha’mazon or repeat the Beracha of "Ha’mosi" in such a case. This ruling appears in Yalkut Yosef – Berachot (vol. 2, p. 78; listen to audio recording for precise citation), and in Hacham David Yosef’s Halacha Berura. Nevertheless, Hacham David adds that if one wishes to follow the view of the Shulhan Aruch and recite Birkat Ha’mazon and "Ha’mosi" before resuming his meal, he may do so. This is mentioned as well by Hacham Ovadia himself, in Halichot Olam (vol. 2, p. 40, in a footnote). But as for Hacham Ovadia’s actual position, he rules that one should resume his meal without reciting any Berachot, in light of the different views that exist regarding this issue.
Summary: If a person left his home in the middle of a meal that included bread, when he returns he continues eating without reciting Birkat Ha’mazon or any Berachot. This applies even if he was eating alone and did not leave anyone behind at the table when he left.
