What makes sentinels hostile?

Sentinels will only attack if you are attempting to make changes to the area they are residing in, provoking them by messing with the wild life or terrain, or exploring a planet with rare resources.

No Man's Sky - Sentinels

Summary

The Sentinels in No Man's Sky are a species of self-replicating, non-organic machines. They were created to balance the efforts and actions of the player with the procedurally generated ecosystem. Sentinels act as the workhorse machines of The Atlas, and police the actions of those within the universe.

They are, in essence, the "gardeners" of the No Man's Sky universe. If the player makes an attempt to alter the environment while they are around, they will not hesitate to defend their "garden".

Behavior and wanted level

Sentinels will normally act passively towards the player unless they witness them violating a universal law (such as killing an animal unprovoked, or over-mining a planet's resources). This behavior will accrue wanted levels. The aggression of the Sentinels and their dedication to pursuing the player is entirely linked to the player's wanted level. As this level increases, the number of Sentinels will increase, and more dangerous Sentinels will also start to appear.

On high security planets, because endangered resources are likely, sentinels will attack on sight.

In Addition, the following will cause sentinels to appear and attack you - even when there are none in sight:

1. Picking up Gravitino Ball from the ground

2. Picking up Sac Venom from the ground

3. Taking a Vortex Cube from a pedestal. Picking up a Vortex Cube from the ground will not alert sentinels if no sentinel witnesses it.

4. Removing an Albumen Pearl from a shell

5. Attempting to destroy reinforced Resource depots or steel gates of a Manufacturing Facility

• Resource extraction
• Looting Depots and Manufacturing facility's
• Hostility towards sentinels, living animals, freighters and other star-ships
• Repeatedly attacking space stations
• Being on a planet where sentinel behaviour is erratic or hostile
• Picking up rare resources like a Gravitino ball will instantly get you a level 3 grade

These will all cause sentinels to attack the player.

I may have missed some things but i hope this helps.

Each planet has a hostility level for sentinels. Though not all planets seem to have sentinels on them, at first. (They may only appear at specific "rare elements or special objects or locations, such as bunkers or stockpiles.)

If they are "Aggressive" or harder... They will attack you just for being alive. You don't have to "do anything", to make them want to attack you. Contrary to what the first answer suggests, which is wrong. If they see you, seconds later, they will start attacking you.

TIP: On those planets, travel underground. Do all your scanning from the caves you dig, or find. You can do everything you need, from the protection of underground.

HINT: There is a trick to navigating underground well. However, it uses a slight glitch in the game. If you dig a tunnel, or find a low ceiling in a cave... You can walk into it, and see through the "ground" or "wall", while pressing forward and looking around a little. You should be able to see more caves which you can connect to, for faster traveling. You can also see resources and buildings on the planets surface too.

There are tricks to manage every type of planet navigation. Even the most hostile. If you want a cake-walk, play in creative. All other modes will have some kind of challenges.