On numerous occasions I've approached a quest marker only to find nothing or no one there. If I pass the exact spot of the marker it scoots way past me on the pip boy. Any explanations out there? Thanks.

If it's in a building they could be on another level or possibly it's indicating a doorway you need to go through.

Sometimes the markers are just plain wrong (not very often but it does happen).

But make sure that you have the quest actually selected

i find it usually happens after a fast travel. reloading the cell usually fixes it for me.