What does red shield icon on lower right mean?

In No Man’s Sky, sometimes a red shield icon will appear in the lower right of the screen. What does it mean?

The most obvious answer is that it means my suit’s shields are down, but all my gameplay seems to contradict this guess. I’ve played maybe 300 hours in 2 years, and I’m still not sure what it means. I have definitely had my shields damaged and not seen it, and I’ve been almost certain that my shields are full and seen it (as in the screen shot).

Solution 1:

Based on this guide, when the backpack icon turns red it simply means that all the slots in your inventory have been filled. You can still pick up stackable items (assuming the stack limit hasn’t been reached), just not pick up any new items

As you fill your suit's inventory, the bar on this icon will move up until the icon turns red, indicating you've run out of inventory slots.