We haven’t. It’s been years.
Oh, should have read down. It’s been years, you say. Hmm, after X amount of time, it may have already been turned over to the government. Check your state’s unclaimed money and property. I take it you don’t have his statements anymore? Then start calling each of the banks he used over the years. On second thought, go by each of his banks in person so they have to deal with you face to face. It’s too easy to poo-poo you off on the phone. It’s unlikely he had a box that he didn’t bank at. If you can’t find it, then tell yourself he emptied it decades ago and forgot to return the key so you don’t worry over it.
If it’s been years since the bill was paid, the box has likely long since been drilled and re-let. Check abandoned property records for the state in question to see if anything was turned in.
And if it’s been long enough, the physical bank or branch might not even still be in existence. If you want to track it down as a hobby, have at it, but you are unlikely to find a pot of gold at the end of the road.
That being the case, go to your State's website and find the 'unclaimed property' section (I think every State has one).