this sounds like a promising tip. maybe Holocene or someone with a connection to the family/investigation could pass it along to the appropriate people. they might already have it, but it's not something i've seen mentioned anywhere else.
cuz if the scenario were to be something like DS gets into a car with her eventual abductors, and she has her cell-phone with her and fitbit on-- say they drive her to some other location before they take away her cell-phone and maybe take away the fit bit, the fitbit could have registered that location or locations along the way.
of course then they might well be able to track the cell phone pings, if it turns out the burner phone she had is trackable. but i believe the fitbit works off of GPS, not just cell tower pings; which should provide a more accurate location.