Your main character is asleep and though it is early, the sun is lightening the room enough to discern furnishings and objects about the place. Have your main character begin his usual wake up routine. When he gets up to sit on the side of the bed for the last residuals of sleep to pass, have him notice something in the room that is just not as it should be. Maybe someone else's shoes are next to the bureau or perhaps different jewelry is in the tray where cufflinks or earrings are normally left to be put away later or used again. Maybe the bedding is not the same as it was the night before. Whatever it is that is different, have your character figure out why it is.
(To avoid the he/she, his/her, etc., inserts to avoid saying "they," I put a male reference and for no other reason. Replace it with a female reference if needed.)