From Consumerist, a cautionary tale of one man's attempts to access his late mother's safe deposit box:
My mother passed away in October last year, and she had added me as a signatory on her checking and savings account the day of a heart procedure…a day before she passed. She did not add me as a signatory on her safe deposit box, likely because she didn’t realize it was a separate form to fill out or because she forgot all about it. ... I am the sole heir to her estate, and listed as the executor of her will and sole heir. I’ve worked with a lawyer who helped file an Affidavit of Heirship with the court, so I’m covered there. Read more
.... My expectation is that there’s nothing of value in the safe deposit box…maybe the original of her will, which we don’t need, maybe a social security card, things we don’t need. The sort of stuff that someone else might have in a small fireproof safe in their house.
What's in the box? No one knows, but the estate may ultimately have to go through probate at a cost of approximately $1000 just to get the exact paperwork demanded by the financial institution in order to be allowed to open the box.
It's a good idea to keep an inventory of safe deposit boxes with other important financial documents.