It's a secret...but the fact you got evidence on him is also a secret. So when he starts to get suspicious about why you spent 7 hours at the hair dresser, about why you stayed overnight at the grocery store, about how you went to your parents' for dinner and your parents called to say hi to you while you were eating with them ..... It is then that you throw your evidence on him back in his face! It's not petty. Or maybe it is, ionno. But it's how things are done.