As a teenager she would 'sneak' out (of her house/apartment) at night to visit her boyfriend.
I'm going to 'sneak' a smoke. [=I'm going to smoke a cigarette while nobody is watching]
She took a 'sneak peek' [=a secret look] at the birthday presents hidden in the closet.
(..) Let her stay up all night crying and writing to the parole board. What am I doing here? Where am I? I'm in jail. I can't stop people from visiting me. Let her sneak this stuff in for you every week. Let her fight these bastards every week! Look what you're doing! Stop it! I'm sorry. Let her sneak this shit in for you. Will you stop it? - Let her do it! - Stop it! Shh. Ruth, Ruth, come here. Nobody is helping me. (..)1990 Goodfellas