the fitbit type devices piggyback on the GPS of a synced device--typically one's cell phone, but you can sync them with computers or anything with blue-tooth and the ability to load the fitbit software as well (work computer, tablet, etc)
If she had a fitbit and she had it synced to her cell phone (and it had GPS enabled), and she still had both on/with her when she disappeared, one could either download the fitbit app, or maybe via their website, and login*** under DS's account and the app will have a "locate my fitbit" function. That will tell you the last time it was "seen" by the tracking software. that is, where DS's fitbit was located the last time DS's fitbit--if she had it blue-tooth synced up to her cell-phone--was in communication with the synced device.
considering she had a "burner" phone, she might've been syncing it via her work laptop/computer or a home computer or tablet. but i have no idea what capabilities her phone had, other than it's being termed a "burner".
*** if you either know or could guess/hack her login and password, likely her primary email address for username. LE might also be able to get FitBit to provide that info, either with or without a warrant. Heck, fitbit could likely provide a log of all synced locations for her fitbit over whatever time period LE was interested in.