Just reading through that, the unanswered questions are literally the same questions asked before the research began.
Their paper doesn't identify where the signals came from. "The emitter source remains an open question, but could range from covert ultrasonic exfiltration of modulated data to ultrasonic jammers of eavesdropping devices or perhaps just ultrasonic pest repellents. It’s also possible that someone was trying to covertly deliver data into a localized space using ultrasound to say, activate a sensor or other hidden device."