The Safe Room by Lisa Unger

Private investigator Rae Donovan’s relationship with her father is distant to the point of estrangement. But when she learns he’s dying in hospice, she drops everything to be at his side. By the time she arrives, he’s already too far gone to speak, and a doctor says his death is consistent with poisoning.
As she scours his isolated Adirondack home for clues, she discovers a hidden room filled with surveillance equipment, weapons, and a safe full of cash—maybe her father wasn’t the unassuming retiree she’d imagined. Rae must decide whether knowing the full truth about her family is worth the danger of tracking it down.