sneak up on
sneak up (on someone or something)
To approach someone or something in a sneaky, furtive manner so as not to be noticed. Don't sneak up on me like that—you frightened the life out of me! We don't want the guards to see us, so we'll need to sneak up from the back.
sneak up on someone or something
to approach someone or something quietly and in secret. Please don't sneak up on me like that. I sneaked up on the cake, hoping no one would see me. someone did.