Many thanks to the lovely folks who wrote me off-list, pointing out that
my mystery plant is Vicia faba, fava bean! It certainly bears no
resemblance at all to any species of Vicia I have ever seen in the wild.
I think the folks who place it in a separate genus might have
something....

Monique Reed

monique wrote:
This plant has come up in a compost heap. This heap receives the
leftover props from a college Economic Botany course, so it might be
anything. The foliage looks very much like regular Pisum sativum
(English pea) foliage, except that so far, there are no tendrils--and
the plant is over a foot tall. The leaflets are thin but a little
rubbery and glaucous. The flowers are about an inch long, white with
maroon blotches on the wing petals. No fruit yet.

http://www.csdl.tamu.edu/FLORA/tfp/mysterypea.jpg

Thanks,
M. Reed