Some will likely say "Well she should have left a note"--but there are times when even leaving a note could be dangerous. For example, if my husband were an abusive jerk and I knew I needed a head-start of at least a couple of hours, I'd leave without a note--that way when he got home, he might think I had just gone to the store or something, and wouldn't be chasing me yet. Besides, even if I *did* leave a note, the person I'm leaving could easily toss the note in the trash and report me missing out of spite (knowing the police will help him find me, and I could potentially get into trouble for the money they spend searching for me). How would I ever be able to prove that I left the note in the first place?
What about sending a registered letter? It would probably take more than two hours for it to be delivered and you would have proof that you sent it. A registered letter could contain any note that you wanted to leave.