It's a 6930, so instead of lifting the receiver and pressing the VM button, just press the message button and READ the message to see what left it. If it's the embedded VM, then look for it's mailbox in the embedded voicemail and delete it, or export the embedded VM form and search for that users extension to see if it's associated with a mailbox that isn't it's extension.
If this was a message in the mailbox, when you disabled the light it would be back on every morning... The embedded VM runs a routine at about 3-4am every day to refresh the message waiting lights, if there is still a message in there it would come every day until it's cleared out.