I think, we don't know:
at what time the witness saw something
at what location the witness saw something (near trails/near water/near any bridge/near street/near building)
whether the witness already knew about the missing girls, when s/he saw something
whether the witness already knew about the found/deceased girls, when s/he saw something
how the person seen was dressed exactly from head to toe
if any car played a role, when the person was seen (ie standing near a car, entering/leaving a car)
if the person was seen walking (gait!) or standing/sitting still
what the person did exactly - we know: something striking unusual/criminal (would help immense to know that, IMO)
whether the person had at least one animal, ie a dog, in his company (here taking some distance from assuming a possible goat
)
Idk, what else could be, that we don't know. In any case, if we would know details, the connection with the young face of the YBG suddenly would be more clear to us, I believe. I guess: we the public shouldn't be able to know more at this time; otherwise the further investigation would be impossible.