The problem with notes left on your car is that they let robbers know how long they have until you return.

The method I use is to give a friend or love one a detailed plan of who is going, where they are traveling and when you expect to return. Let them know you will notify them immediately upon your safe return and give them instruction to call the authorities if they don't hear from you.

Also sign in at all registers and leave a detailed plan at the local USFS office.