As you are exploring the first biome in Returnal, you will notice large gates that look a bit like drawbridge gates. Light often beams through them and behind the stone cage-like bars will have a chest that should contain a weapon or some good items. How do you unlock these gates? Here is the answer.
You will need to look around the area for a sort of pyramid-shaped button with an orange light on the top of it. They are rather large and hard to miss but they are hidden in locations that are out of view so you will need to look around to find them. The button location for the gate from the image above is to the left behind a statue.
Once you locate the button, shoot it and it will cause the nearby gate to slide up, giving you access to the chest or items that are stored inside. The button is always nearby and in the same area as the gate. You will never need to look too far but they are very well concealed from view. It is best to take a few steps back and look around from a distance. This will help you spot the button to unlock the door.
Keep in mind that weapon drops from these chests are based on your combat level at the bottom left of the screen. It is no harm killing some enemies before unlocking these chests so you can find some weapons that will last you a little longer.