Where's the Phoebe Nest?

I hear a constant peeping right above my apartment and the constant call of a phoebe nearby, but I don't see any nests. Phoebes like to nest on the edges of buildings near overhangs and the like. Sorta using the building like a cliff. I do see the Phoebe flittering around the building next door, but I haven't found any nests. I hear a constant "peep" "peep" all day long. Also, there is a concentration of bird excrement in one area near a tree, but I still see no nest, phoebe or otherwise. I'm guessing, also, there's a house finch nest (or several) nearby, too.