This will be caused by rust coming through from the inside. In this case between where the inner and outer sill join.

Because it's already started (and the majority of it is not seen) it will be difficult to stop, but you can buy some time. Flood the sill with your rust treatment (phosphoric acid, that's what turns it black) and then flood the sill with cavity wax. Do this till it drips out all over your garage floor (get the wax hot so if flows better, a nice summer day is perfect along with keeping the can in hot water) it will wick into any panel joins and seal it from moisture and air.