It's called an indefinite or generic use of you.
It's equivalent to the more formal indefinite pronoun one:
- You never know what you'll find there.
- One never knows what one will find there.
or other indefinite NPs, like
- People never know what they'll find there.
In all these examples, the subject NPs don't refer to any specific individual,
but rather to anyone who fits the description in the sentence.
In my experience, the use of the indefinite you is especially frequent in giving advice,
where the you can refer either specifically to the addressee, or to an indefinite person,
or to both, which makes it convenient.