Could your mystery plant be the one shown on Margaret's site: http://awaytogarden.com/what-weed-is-it-putting-names-to-pesky-plants?
...this is the one in our flower bed...
At first, I thought the leaves on my plant were more pointed than the one on Margaret's site.
But after checking out images of garlic mustard on the internet, I was quite sure that's what it is!
After smelling it, I KNOW that's what it is!
Gardener on Sherlock Street certainly lives up to her name! Thanks, my friend!
Garlic mustard is an herbaceous, biennial forb that is an aggressive invader of wooded areas throughout the eastern and middle United States. First-year plants are basal rosettes with green, heart-shaped, 1-6 in. (2.5-15.2 cm) long leaves. Second-year plants produce a 1-4 ft. (0.3-1.2 m) tall flowering stalk with small, white flowers in the early spring. Plants can be easily recognized by a garlic odor that is present when any part of the plant is crushed and by the strongly toothed, triangular leaves. A high shade tolerance allows this plant to invade high-quality, mature woodlands, where it can form dense stands. These stands not only shade out native understory flora but also produce allelopathic compounds that inhibit seed germination of other species. Garlic mustard is native to Europe and was first introduced into New England during the 1800s for medicinal and culinary purposes. (http://www.invasive.org/browse/subinfo.cfm?sub=3005)