Just in case anyone is still reading this post, you will probably find that the zener diode for that floor has started to get week and is passing enough power to fire the call. The zener boards typically have what looks like 8 regular diodes in a star configuration. You can't test a zener with a multi-meter, so to test them, just swap the wires from those floor buttons (car, hall-up, and hall-down) with a different floor and the ghost calls should follow the wires. Or, for the cost, just replace them.
Good luck!