I’m not sure you understand the term “take it to the grave”.

My secret is that I know I’m actually the only real human, and everybody else are aliens posing as humans to study my behavior. That’s why I purposefully make random decisions and actions from time to time, to throw them off.

I still have to figure out if I’m the last human alive, if every remaining human is being studied like me, or if there is a real human society somewhere.