1.
If someone sneaks up on you, they try and approach you without being seen or heard, perhaps to surprise you or do you harm.
I managed to ~ him when you knocked on the door.
= creep up
PHRASAL VERB: V P P n
2.
If something sneaks up on you, it happens or occurs when you are not expecting it.
Sometimes our expectations ~ us unawares.
PHRASAL VERB: V P P n