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.
(..) Good. Then it's my problem. - How are you, Mr. Corleone? - How are you? - Good. How are you? Good party. - You like it? - Yes, I had to sneak in. - You're dressed for it. So, what's the trouble between you and Mr. Joe Zasa? - Just trouble. I'll take care of it. - That's foolish of you. Vincent... (..)1990 The Godfather Part III