You're absolutely right about being most likely to find things concentrated at the narrow points of a funnel shaped field. This is why prospectors pan for gold in the streams rather than sifting thru random shovels full of dust on the mountainside.
My original comment was tongue in cheek....An order-of-magnitude estimate-- Suppose a village consisted of 200 people, half whom had amulets of which only 0.1% were lost each year over a 500 year existence of the village-- That's 50 amulets for archeologists to find today at that one site....and how many sites are there? It adds up fast.