jpk0104, there are several ways to light someones corner lamp. Voicemail would be one of those. Leave the user a voicemail and have them check and clear the message and see if voicemail will turn the corner lamp off.
The other way is if you have a 60 button DSS Console at the receptionist. It may have a MW button programmed. It might not be labeled and would most likely be on the bottom row of buttons. This toggles the DSS Console into a MW console. If in this mode, if you see any button lit green, then the console operator set a MW to this extension. The only way to clear it is from the DSS Console using the MW toggle. Press the MW button and then press the DSS key that's lit green.