@eurocelticyankee,
I am going to do some digging literally this weekend.
At first I thought maybe they were those you use for filling glass vases - and maybe the former owners had a flower basket or something hanging - but it way too far from the house for that -
And then I thought maybe when their boy was young he used marbles - but that didn't make sense cuz they are all the same color. A kid would have a variety of colors.
The Leprechaun thing makes the most sense right now. I mean we are on the edge of the woods - maybe like the animals - building so many homes has displaced these woodland fairy type creatures.