It feels like that'd really boil down to whether identifying a fugitive's location based on signal from the location-broadcaster they're carrying voluntarily on

It feels like that'd really boil down to whether identifying a fugitive's location based on signal from the location-broadcaster they're carrying voluntarily on their person, connected to a distributed service network shared by customers of a private company, could be considered "unreasonable search and seizure." To a first approximation, my guess would be "probably not." No more so than, say, a train station dropping a dime on you if you try and buy a ticket on a train and your name comes up as wanted.