Sorry to hear of your and his wifes loss.
I would have thought you would come across the keys whilst helping his wife sort through some of his shooting gear ?
Then get a friendly rfd to store them or sell them on, whichever is decided.
Incidentally this is one reason my wife now has a sgc, could save a lot of hassle and money in the long term, worth £50 for peace of mind ?
Althougfh I know she will sell the lot......lolol, except maybe my sons.
Dave B