This is a common mystery, Kalanchoe blossfeldiana or Plectranthus verticillatus. Kalanchoe leaves are smooth on the back, Plectranthus leaves have prominent, raised veins, like this:
K. uniflora does seem to be a valid name, but this discussion was my first time hearing of it. K. blossfeldiana is ubiquitous, sold by the millions every spring. Much more likely, IMVHO, and that's what it looks like to me.
Kalanchoes usually bloom about this time of year. Plectranthus usually starts blooming in the fall. Blooms would definitely solve the mystery.
I have put many pics of both plants in the database here.
Florist Kalanchoe (Kalanchoe blossfeldiana)
Swedish Ivy (Plectranthus verticillatus)
