From reading (ok, skimming) the paper, it seems that the sensor is powered by an external ultrasonic transducer and, I think, returns data by modulating that signal (a bit like RFID). That means that you lose power when the ultrasonic is removed.

It seems that you could do a lot more with it if you added a battery and some memory so that it can record signals when the ultrasonic is not present? I am thinking of things like the Michigan Mote which was also about a cubic millimetre. http://www.eecs.umich.edu/eecs/about/articles/2015/Worlds-Sm...