I think they both use their own, but active at the same time as they are located slightly over a mile from each other. Since the fog sensor activates the horn via radio, that totally can be the case

@@SirensAndAlarmsOfNorthernIL There is no fog detector located at this lighthouse, in some cases where there is no sensor, the foghorn will be triggered 24 hrs a day, but, I read that the one at Owls Head triggers both foghorns at Owls Head and this lighthouse. Of course the US is now using a different system as apparently the sensors are too expensive to repair when they are broken, so the horns are now triggered via radio link, something called MRASS, ships will call a number to activate the horn and it will sound for 30 mins. I have a feeling that that may cause a lot of problems.