I'm wondering about the context of that statement. If she just out of the blue said "If I/m not back by 6 call the cops" I'd think that would have raised a huge red flag and there would have been further questions.
On the other hand, if she was headed out the door and someone asked where she was going/with who/would she be back for dinner, then I could picture her answering with a sarcastic, flippant "If I'm not back..." without that causing any alarm. If it's the latter, it makes more sense, and I wouldn't read much into it.
I'd also hope that her family has checked for any missing valuables (jewelry, etc) that she might have taken with the plan of pawning/selling.