If so, you may want to avoid it in the future. It turns out that the park used to be used as a Potter's field in the early 19th century. Lest you think that the bones would all be gone by now, it appears that they recently found bones there and during a previous recent excavation as well.
Of course, the idea of long-forgotten cemeteries is not new. The African Burial Grounds in downtown Manhattan were only rediscovered in 1991 after having been lost for over a century. I recently saw a monument in a children's playground/park in Greenwich Village saying that two fallen fireman were buried there in the 1840s. As it turns out, the monument had been moved from the actual cemetery years ago.
Considering the city's diverse, long and sometimes obscure history, how do today's Kohanim keep track of where they can and cannot go? I assume it's probably OK to go somewhere until a cemetery is found, but once it's found, do they keep a list of such places?
Disclaimer: Of course, this is not a p'sak. I am not a posek. Make sure you discuss matters with a competent rav before following any course of action.