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I can understand how difficult this might be locating relatives you have had very little contact with, as I have numerous cousins who I have no idea where they are located.... but I have been in contact forever with some of their siblings, so one quick phone call I could have my information.
As to the bank accounts and insurance policies, were they held jointly with you, and if so, were they payable on death to you as a survivor? Are these the assets that concern you in terms of contacting the beneficiaries?
As for the insurance, it isn't up to you to find those lost beneficiaries, but you should notify the companies of his death and provide a death certificate if you can. Perhaps they are honourable enough to do the right thing and return the money to the person it should have come to, if not you will have lost nothing by making a few phone calls and doing a little detecting to try to find them.
If the document is not able to be viewed as a will, I'd bet that you will have to do a lineal heirship to disburse any of his assets. Lineal heirship will need some sort of searches done, notices posted so it too is not a DIY project and needs an atty.
But neither needs to be done today, tomorrow or even this summer.
One thing you will need no matter which you attempt is death certificates. I'd suggest that you get at least 10 death certificates. Usually the FH orders these for family initially but you need to let them know the amount. The more you order initially the cheaper it is and quicker you get. Every possible insurance company, bank, etc will want an original certificate to do anything.
His bank accounts, unless they were POD to you or you were a co-owner on the accounts with your name & SS# tied to accounts, they are going to be frozen. The banks have a program of notifications from SS as to death. So his accounts will be frozen once that happens, although his SS or other retirements will be clawed back for the month(s) of overpayment. It happens within a couple of days. If he was paying for things on autopay from bank accounts those will be rejected for payment, or if you need his funds to pay for funeral & burial that can't happen once it's frozen. If you start paying for things, it's crucial that you keep all receipts& documentation If you want to be able to be reinbursed from his estate as a claim and ahead of the heirs being paid.
The insurance co won't find beneficiaries. They may not even know if he died for a while. But what will eventually happen is that the unclaimed funds that would have been paid IF the beneficiary had contacted the insurance co will instead go to his state Dept of Insurance unclaimed funds account. What then happens depends on your states laws & how your Insurance Commissioner is inclined. Some states do a annual publication in major newspapers across the state of names and amounts, which owners or their heirs can then claim. My state -louisiana - has a new insurance commissioner (kennedy) who has done a major outreach to get unclaimed $ back to owners or heirs; it was at Lakeside, the major mall in New Orleans area and lines were out the doors, quite amazing. Other states don't and after a long period of time, all unclaimd funds escheat to the general fund or insurance dept budget.
There was an especially moving article on finding beneficiaries by the great metro editor NR Klienfeld in the NYT....October 15, 2014 - "The Lonely Life of George Bell". Quite a detailed story of how the Queens co. public administrators office dealt with finding assets, dealing with old insurance policies & beneficiaries.