If you really want to know the source of the light, you need to use a station with global message retrieval. Do a 'change system-parameters features' and add a display phone to the list of stations with global message retrieval on page 2 to program one.
Then add the following buttons to the display phone: cov-msg-retr, next, and delete. Press the cov-msg-retr button, and dial the extension of the phone with the stuck on message waiting lamp. Press 'next' sto scroll through the messages for that phone.
If you see a message from another station, press delete.
Press exit or normal to clear the display when you're done.
Of course the 'clear amw all <extn>' command will shut off the message waiting lamp, but it won't tell you why it came on, or if it will happen again.
Carpe dialem! (Seize the line!)