True the rental fee is a good indication, however, the box could still lay dormant for 20 years before the alarms goes off.
Case in point.
I had a checking account at a bank in my college town while I was in school. I also had a safe deposit box associated with it. When I graduated and left town, I forgot to close my account. I had $212.00 in it when I left. The rental fee ($14.50) was deducted from the account each year. It took the bank 14 years to figure out that the account and box was dormant. As long as there is money in the account, a box can lay dormant for decades without alerting the bank to the fact that the person had died or has moved away and is not using the box anymore.
Lucky, for me, I was not dead and not hard to find. The bank contacted me and I went to see what I had left behind. I had left behind a few rolls of silver dollars and a Gretsky rookie card. I bought the card for $275 and by the time I was able to retrieve it ..it was worth about $780. I sold it for $720 to a guy in a card shop a few doors down from the bank.