Please note that one will usually have to get a court order to open a safe deposit box if you are not on the signature card. I have known some people who kept many important papers in the glove boxes or trunks of their cars. Make a thorough search of all vehicles. People I know who conduct estate sales report finding all kinds of interesting papers in the pages of books. If you have run out of ideas as to where the will could be, try going through the books to see if the will has been stuck between the pages.
Some people have wall or floor safes.
Those are obvious places to put a will. Make a thorough search. Many times the will is left with the attorney who drew it up. How to find the name of the lawyer is the problem. If you find a business card of a lawyer be sure to call that lawyer or law firm.