According to the world of Twilight, keeping the existence of vampires secret is crucial to any vampire’s survival. The Volturi, a large coven of Vampires who reside in Italy, are recognized as the official leaders of vampire society and enforce the rules that are used to maintain secrecy.
Vampires in the Twilight universe are expected to follow the Volturi’s laws, and those who break the rules are typically punished with death. Such rules included avoiding direct sunlight, since vampire skin would sparkle and quickly draw the attention of any humans nearby.
However, despite the Cullens carefully adapting their lifestyle to keep a relatively low profile within Forks, there are still plenty of times where a reasonable person should have said, “Hey, those Cullens are a bit off.”
For example, while the Cullens explain to Bella that they settled in Forks because of how constantly rainy and cloudy the climate is, there are still a few times that the vampires were exposed to direct sunlight in front of other humans. When Bella sees the Cullens for the first time, they all sit down at a cafeteria table together by a window, where a tiny bit of sun filters in through the glass, and you can see a bit of light on Alice’s arm. You’d think that for people whose lives depend on avoiding the sun they’d sit further away from windows.