Haha! I just recently came across a planet with a bunch of Vortex Cubes, but mine has the red box hovering over it like this:
Such an easy farm, but yes it is really easy to get greedy and get killed by sentinels. Just taking one of these Vortex cubes calls in two floaty sentinels and the mechanical dog, both are easy to take out but its easy to get distracted taking so many of these things as loot.
Weird. I found a vortex cube planet but they were in caves and just a small trapezoid-y metal box. And they didn't trigger sentinels.
Later I found a Graviton Sphere rich planet and it did alert sentinels (3 stars) but I'd just keep moving and kept picking up more Spheres and only came close to dying when there was a radiation storm and I was far from my ship.