A Mountain Locator Unit only transmitted a signal and did not initiate a rescue (when you activated an MLU beacon, there was no one monitoring for signals, the device only assisted rescuers in locating lost climbers once a rescue has been requested by other means and rescuers know to listen and search for the beacon's signal.
A Mountain Locator Unit only sent out a signal but did not start a rescue operation. It assisted rescuers in finding missing climbers only when they had been notified of the beacon's presence and had initiated their own search.