The track had been powered up and I was running another loco. The RS11 was on a powered track that is not part of the main loop. I had never addressed the RS11. When I have addressed it I do a shut down. It happened once this way also. I have seen it do it 3 times total. It only moves an inch or so.
The track it sets on is a connection of two switches on opposite sides of the loop.
It is not a problem unless it moves into the switch and interferes with the other loco.
I can put some insulated pins in and power down that part when not using but I thought I would check here 1st.
I have parked 2 other locos in the same spot while running and they did not move.
Don