They attack if they are ready and engaged, just like any other enemy. But not only won't they attack you when they're not engaged, you can't attack them unless you engage them.

Do aloof enemies engage?

Aloof is a keyword ability. An enemy with the aloof keyword does not automatically engage investigators at its location.

Do Aloof enemies hunt?

Erik O'Rourke. if you've read the card, you'll see that if you gain clues while Pit Boss is at your location, he will engage you and attack you. aloof in no way, shape, or form stops the hunter keyword.