I have heard the same single-drop sound. It's almost like a recording of a water-drop sound effect: "bwop." It's from the rear right part of the car. It comes only sporadically, and it's too loud to be an actual fluid.
Did you ever find out? I'm having the exact same issue: A recorded-sounding electronic "bwop" sound (like an old Apple Mac "droplet" sound) that comes from the right rear on my 2007 Prius, which currently has 95K on it.
It's sporadic, and I can't figure out it's relationship to braking or inclines.