The Levite only mentions himself and his concubine in his report. It may very well be that he omits mention of the slave or the old man in order to avoid having the assembly think of calling them as witnesses. He does not want them as witnesses, because they could have discredited his tale.
4 And the Levite, the husband of the woman who was murdered, answered and said, “I came to Gibeah that belongs to Benjamin, I and my concubine, to spend the night.