The death of Frank Mathers is a mystery in Grand Theft Auto V.
Frank's wife, Abigail Mathers, can be found at the Sonar Collections Dock. To activate her mission, the player must purchase the docks for $250,000. When Michael first meets her, Abigail talks about the recent death of her husband, who drowned, while he was testing a submarine. Michael mentions rumors that someone tampered with the airlock, which Abigail quickly belies, saying that was never proven. She then asks Michael to find all the pieces of the submarine, so she can prove that Frank is dead, and collect his life insurance, and also be able to sleep much easier.
When Michael comes back to her with all the Submarine Parts, she's surprised to find a piece of evidence that suggests sabotage, she would claim that the police are in on his death, and intends to take her findings to the FIB. She rewards Michael with a mere $10, and a rare signed autograph from her husband, much to Michael's chagrin. The player then has the option of killing her, or just letting her go. As the player pursues her, she'll still claim that she didn't kill her husband. Unless, if the player cautiously follows her to the city without ramming her and triggering a chase, her destination won't be the FIB but rather the airport, where she's more likely to skip town, which might mean she killed Frank Mathers to claim all his money for herself. At this point she'll get out on foot, and scream for help, claiming that Michael is harassing her, thus, triggering a 3-star wanted level.