I noticed this was your very first post - so, Welcome Aboard! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forums: Select Your Area.... and selecting location information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country).
I always keep an eye out for arrowheads and find them in the strangest places... For instance, found several last night in a bag of tortilla chips. They're everywhere!
One other explanation is possibly another hiker just tried his hand at amateur knapping and thought it’d be funny to leave it where someone would find it.
Instead of an arrowhead could it be a club? I know it isn't worked like flint but I've seen clubs where the head wasn't worked. Some with just a river cobble tied to a stick.