Plant ID is difficult without blooms to assess, and in this case, several look-alike vine species have heart-shaped leaves like this. Might a neighbor on the other side of the fence be growing Pipevine? Although its native range in Maryland is only in our far western counties, it has become popular in butterfly gardens (as a host plant for Pipevine Swallowtails) or for use as a deer-resistant vine to cover a support for aesthetic reasons, so could readily appear in other parts of the state. Like many other vines, it can sucker and might emerge from root sprouts several feet (or several yards) from the parent plant. One reason we suspect these are suckers from an existing vine instead of from a seedling is the fact that it only appeared in your garden late in the growing season and presumably grew pretty fast instead of beginning with seed leaves that a newly-germinated plant would typically have.

If you rub/scratch/crush the leaves, do they have a pungent odor? Pipevine usually has a prominent smell when bruised.

The leaf veining doesn't appear to be as good of a match, but Anglefruit Milkvine and Honeyvine also has heart-shaped foliage. However, these species has oppositely-arranged foliage while Pipevine has alternately-arranged foliage.

Miri

Thank you for the rapid response Miri,

Our apologies for the very slow reply...a system glitch in Ask Extension caused us to receive client replies quite late. Morning Glory could be a possibility since that was in the packets used, so perhaps that's the correct ID. If the plant resprouts from the same root system next year, that may suggest it's a Pipevine or another species, since Morning Glory is not winter-hardy here except to germinate from seed the following year. (If this plant didn't flower at all in order to set seed, though, then it might not come back at all.)

Miri