The gun and the thumb

First of all, (I believe ) that nobody, except maybe his wife, even knew that he owned a gun. But his wife is dead. She'll never tell. Why help find the gun? If found, why say "It's mine?" There were no fingerprints, nothing to connect him with it, in any way. He could have tossed it in the sea, at a later time. Even if someone did know, you can't connect a nonexistent gun with a murder.
The thumb: He almost got hanged, because he hid it too well. He cut it so close, that in another two seconds he would be dead, no matter what his ex-partner found. The writer was playing with us, with a goal-line tackle.