According to my research, it is not entirely dangerous for mayonnaise to sit out if it has been opened. A few hours is not long enough to spoil it. Refrigeration is best to keep the flavor in tact, but mayonnaise is not refrigerated in the store or until it is opened. The vinegar in mayonnaise does not go bad and acts as a preservative to the eggs or dairy the mayo contains. However, any mayonnaise that smells sour or looks runny, curdled or discolored should be discarded immediately to avoid food poisoning.