As we’ve discussed in previous editions of Daily Halacha, if a person leaves his home during a meal, he will, under certain conditions, be required to recite a new Beracha when he returns and resumes eating. This would depend on what kind of food he is eating, how much he ate, whether or not he was eating with other people who stayed behind, and so on.
There is a fundamental debate among the Halachic authorities as to whether these Halachot apply in a case where one left for a brief moment, such as to bring in something from the car, bring something to a person outside, and the like. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933), in his introduction to Siman 178, asserts that the laws of "Shinui Makom" (changing location during a meal) apply regardless of the duration of one’s absence. Even if one leaves for just a moment, this constitutes "Shinui Makom" and he is subject to all the various guidelines that apply when a person leaves during a meal. Rav Yaakob Emden (Germany, 1697-1776), however, in his work Mor U’ksiah, writes (based on the Gemara) that a momentary departure from the home does not constitute "Shinui Makom." Hacham Ovadia Yosef, in his Halichot Olam (vol. 2, note 40), observes that this was also the view of the Ritba (Rabbi Yom Tob of Seville, Spain, 1250-1330).
As for the final Halacha, we apply the famous rule of "Safek Berachot Le’hakel" – we do not recite a Beracha in situations of Halachic uncertainty. Therefore, if one left his home momentarily during a meal, when he returns he does not recite a Beracha, regardless of other factors, since according to some authorities this does not constitute "Shinui Makom" at all. It should be emphasized, however, that one should ensure not to put himself in this situation, in order to avoid this question.
Summary: One should preferably not leave one’s home in the middle of a meal until he recites Birkat Ha’mazon or a Beracha Aharona. If, however, somebody did leave his home while eating, but he left only for a brief moment, such as to get something from the car, he does not recite a new Beracha when he returns, regardless of the situation.