I found an thread online where someone found the same thing when they bought a foreclosed house. Apparently it's a good luck custom (origin unknown) of placing a penny on every windowsill on New Year's day every year.
Another post from an online article in the same thread said the following Sherrie Flick and Rick Schweikert, both writers, moved into their 1880 house two years ago to discover gifts from the sellers: pennies on the windowsills and a bottle of Jim Beam in the kitchen. , supposedly this was a gift of good luck left for the buyers and is a tradition, so perhaps the owner's of the apartment follow the same tradition. In this thread it's mentioned being done by many different cultures including Irish & Italian to bring good fortune and includes placing pennies in other locations as well.