Are the outside shower handles turned off. If not this may cause the effect you are experiencing.
Is there an indicator light at each switch letting you know if the pump is switched on or off. I have three switches on my rig. They are for convenience. One in galley, one in head, and one outside by shower. Activating any one of the three switches turns the entire system on. With the system on activating any one of the three switches turns the system off. With the pump turned on all three switches will light up so you know the pump is on. Like I said I can turn it on outside then go to any switch and turn it off. It doesn't have to be turned off by the same switch that turned it on.
I think some how city water is being directed thru the pump forceing it to turn and sound like it is running. The outside shower valves open may cause this or the city water tank valve is not closed or fully closing.