If a person washed and ate bread without the intention of completing the meal elsewhere and then left his house and cannot return, is that person allowed to say the grace after meal at his new location without eating another kezayis? If eating something is necessary, must it be food that requires a hamotzi blessing before eating, or can a mezonos suffice?
Unless there’s great need you are obligated to return and bench even if you left unintentionally. If you already benched elsewhere bedieved it’s ok. If you don’t have bread and you can’t return (great need) you bench there, if you have bread you eat some hamotzi even if it’s less than a kezais and bench. Don’t say a brocho on the bread once again because of safek brochos lehakel (we are choshesh to the Rosh who holds that since you’re obligated to return you’re meal hasn’t ended in a case of a hamotzi). If you are Sefaradi you do say a brocho again.
Sh”A 184:1,:2, Mishnah Brurah 184:5,:7.