Question the veracity of the story about a missing person and express your doubts regarding the situation.
Something sounds fishy here.  Why would you leave your girlfriend at the beach alone till 9 pm. and after being with her for only half an hour?
"SOME of her belongings, EXCEPT her cell phone, were still on the beach. "   Not a very sensible statement . Something's not right.  Best case scenario is she just said, "screw this jerk who left me out here" and left on her own.