Android - passing data from Service to main at unknown point in time

I've looked at some of the other questions here on SO and none seem to be answering my question. I have a main activity with a simple button. Right now it calls startService(new Intent(MainActivity.this, BluetoothScanner.class) which runs until it finds another bluetooth device. When this is done, I want the BluetoothScanner to report that it is complete and return something (a string perhaps) to the main activity, so I can create a popup or something that notifies the user that a bluetooth device has been seen in the vicinity. How do I best go about this?

The button looks like:

public void onClick(View v) {
    switch(on) {
        case 0:
            startService(new Intent(this, BluetoothScanner.class));
            on = 1;
            break;
        case 1:
            stopService(new Intent(...);
            on = 0;
            break;
       }
 } 
});

There seems to be some alternatives, like global variables (bad), resultReceivers,bundling stuff in intents. But I'm not sure how to do this. What would you suggest solves my problem?

Regards