First, start screen recording from the home screen, then open snap, drag your finger like you would normally do to open a chat, but only do it so much. As long as you don't completely let go and it shows the chat screen your fine. Its the same way you would usually go about trying to make it so people think you haven't viewed the chat yet. Like when your girlfriend is one of those 'Why didn't you respond' type of people you can just say you didn't have your phone.