Q: After marriage, the K1 visa holder left the spouse; whereabouts unknown. More than ninety days have passed and the K1 visa expired with no knowledge of the K1 spouse's whereabouts.
What would be the course to follow? Report this to the USCIS, DOS and how to report it? File for divorce, annulment?

A: Yes, the Petitioning U.S. Citizen can report the spouse to the Immigration Service, USCIS and/or DHS ICE. If they are married, then either an annulment or a divorce should be pursued. You should consult a divorce attorney for procedure.