They wouldn’t need to find the device itself. If you have a phone or tablet and another bluetooth device (speakers, headphones, smartwatch, another phone, whatever,) you can test for yourself how they could work out when the Fitbit left the bluetooth range of the phone. Switch on the bluetooth device, then go into your phone/tablet bluetooth settings and you’ll see it there, offered as an available device to connect to, even if not synced or connected. The phone doesn’t need to have an active connection to the device in order to know it’s there and within bluetooth range, and the phone will have logged the data for when the Fitbit left the phone’s range. They should also be able to tell the rough speed that the Fitbit was moving until it left the bluetooth range, from how rapidly the signal strength from the Fitbit to the phone decreased until the point it stopped, and whether that speed fits that of tne river that morning.