The 18R2 will still show a modules' last known antenna signal strength as reported by the module, even if the module is turned off.
This value will remain in memory until the 18R2 is power cycled. This is because the 18R2 is only updating signal values when there is a change + or -. When the module powers off there is no change in the reported signal number, so the 18R2 does not reflect a value change.