This grows almost ubiquitously in un-managed grass areas. I'd love to know what it is.
If we compare the Wikipedia descriptions of Medicago lupulina (medic) with trefoil there is an interesting comment under medic "The leaflets are hairy, toothed toward the tip, and differ from those of the similar Trifolium dubium in that they end in a short point." The photos seem to indicate this short point, so perhaps medic in this case. Otherwise it appears that the fruits have different characteristics, if you can find any at this time.