I had something similar happen right before my mom passed away.
She was trying to find paperwork to renew the car registration, and I volunteered to check my dad's car. When I opened the glove compartment, a ton of condoms fell out
I got what I needed, pushed the condoms back in and never told either parent. My mom passed away about a week later. My dad started internet dating a week or two after that and moved a woman across country to live with him less than six months after my mom's funeral.
Its been over a decade and I have not said anything to him and likely never will. There was so much more hurt from how he handled himself after she passed and the vitriol that the woman he brought out has visited upon our family. To be happy, I had to make peace that the father I want is not the father I have. I forgive him, but we only have a superficial, occassional relationship. I love him but I know where my boundaries are.
Only you can decide what will give you peace. For me, after years of confrontation I realized dropping the rope would give me the most peace. I am okay with my cordial, distant, occassional relationship. The lack of a grandparent is the hardest, especially since my oldest is just old enough to remember that he was once a very good grandfather.
I have never and will never confront him about what I discovered. I don't think it will cause me anything but more pain.
Good luck OP. Sometimes things really suck.