In the Warmind DLC to Destiny 2, you probably came across some diamond shaped chests “Sleeper Nodes, available after completing the story in Warmind DLC and Ana's Blue Missions.
In order to open these containers you need override frequencies.
There are a total of 40 Sleeper Nodes in Destiny 2, so you need a lot of override frequencies.
To get an override frequency, you must have four resonant stems, which you can get in patrols or public events.
If you have them, then you can convert them to an override frequency. Now you can open any Sleeper Node, here you have to read the hint of the frequency. As you get close to a Sleeper Node you will hear music and have small artifacts.