Welcome to Android Central! There are a number of apps on the Play Store that can hide apps. You could also install a 3rd party launcher like Nova Launcher. I'll leave it at that, since I'd rather not encourage philandering.
Joe, that would let the OP's partner know that he was hiding something but, as B. Diddy said, I'm not in favor of cheating or hiding things from the person you live with either.
For me, passwords/PINs are a matter of proper philosophy. My phone is mine; no one else knows the PIN. My spouse's phone is her phone and I do not browse it. Ditto for account and email passwords. If the relationship has trust issues, knowing passwords and PINs is not going to help any.
I also vote for changing the device PIN/password. It's just good sense to protect your content and make it rule not to tell anyone your passwords. I'd never tell my wife my Facebook password. Then again, I couldn't even tell myself my Facebook password! (Yay password managers.)
My significant other knows all my password, she is not interested in snooping in my device in any case. If I hide something it would be from strangers if the phone fell into hostile hands and she would be aware of this. No trust issues here...