Our mystery "seeds" from last week were from a red maple, Acer rubrum.
The leaves of the red maple vary, with three-to-five lobed varieties.
(Public domain photograph of three-lobed red maple leaves from Wikimedia.)
(Public domain photograph of five-lobed red maple leaf from Wikimedia).
The five-lobed red maple varieties closely resemble sugar maple leaves. The best way to distinguish them is that the the red maple leaves have V-shaped notch between the lobes, whereas the sugar maple leaves have rounded or U-shaped notches (see a photo of leaves of the two species side-by-side at Maine Nature News.)
Red maple seeds do grow from seeds, but may require scarification (scratching) to germinate.
Have you ever seen a red maple tree? Did it have three- or five-lobed leaves?