I suspect I know who left it -- her Mother hung a May Basket in the same spot maybe 40 years ago. And my daughter may have had something to do with it also.
Was (is?) this just a New England custom? When I was in grade school, back in Boston, we were each allowed to choose a single sheet of construction paper (this was during the Depression after all ) and make a May Basket to take home. (Remember construction paper?)
In Louisa May Alcott's book Jack and Jill, a whole chapter is devoted to the girls in the village (certainly Concord), excited about making May baskets. They find only a pitiful few early flowers -- if I remember right, the kindly grumpy old neighbor rescues them by sending over lots from his conservatory.
And when I taught at a college in Maine, in the 1940s, there was plenty of excitement about May Day. With no garden flowers out yet (it was a late spring) students went into the woods and picked early-blooming snow trillium to hang on each others' doorknobs. We can only hope there wasn't any worry, back then, about endangered species -- and that they weren't breaking too many laws!