Is taking a small quantity of food from an open buffet in a hotel, with the intention of eating it later, considered unlawful (haram)? And what should a person do if they leave the hotel and are unable to return the food?

If what was taken from the food is something that is customarily overlooked and permission is implicitly granted for its removal, then there is no blame. However, if it is not so, then you must give it away as charity if it is still available, or its value if it has been consumed, provided that you cannot return it or seek forgiveness from its owners, and they have not absolved you.