Yes, it is parshat Tazria-Metzorah, but I was thinking about this, and felt like writing it down.
One might say, that the Torah records it because it happened, and it is thus rightly part of the narrative. But unimportant details are often glossed over. Why bother mentioning this portion of the incident.
Obviously, there is the midrash on this, that he met the angel Gavriel, and he was hinting/warning that they had moved to judgment.
Two thoughts occurred to me about this.
1. It increases the dramatic tension. Not so much the first time around, of course, because we don't know what will happen. But the second time around, when we have a sense that Yosef is headed towards his doom, or at least years of servitude, if he is earnestly seeking his brothers, and cannot find them, then perhaps he will return to his father and avoid this fate. Yet he diligently wanders the area, and the fields in Shechem, where they usually graze their sheep. And a man finds him, and directs him elsewhere. And then, rather than giving up, he seeks them where they are, troubling himself to travel to another location. Similarly, perhaps it increases the sense of injustice (even on a first read), as he was innocently seeking them.
2. Another possibility is that Dotan was a more remote place than Shechem, and this accounts for why there were no witnesses to the proceeding. (Though it could have said "and he found them in Dotan).
Or to make the cover story more plausible that in seeking his brothers, he wandered here and there and was attacked by a wild animal on the way, before he was able to reach them.