MWI light is on after the phone has been idle for an extended period (overnight) but there is no message and the light quickly turns off after picking up handset. Minor annoyance. Any ideas on what might be causing this?

MWI light is on after the phone has been idle for an extended period (overnight) but there is no message and the light quickly turns off after picking up handset. Minor annoyance. Any ideas on what might be causing this?