. . Wrecks are refreshed at server restart. You may find a wreck with full healthbar that drops nothing. It's because wrecks slowly regenerate their health and someone else looted it before you.
You can loot each wreck only one time per character. If you try to loot it for the second time, it won't drop anything and nobody will be able to loot it until server restart. Make a new character and use that one instead.
Looting wrecks with hazards listed under their note is generally not safe to attempt on the starter ship.
. . Encounter Codenames are dropped from a specific ship in any given Encounter, typically one that's more dangerous than the others.
The specific ship has a low spawn rate, and once it is taken down a set amount of time must pass before it may spawn again.
The numbers between brackets indicate the spawn rate of the ship dropping the Codename.
To find the Encounter area on your map, click the second tab (Political Map) and search for small, perfectly round circles; if none appear, check for them among friendlies and hostiles tabs. You'll know you're in the right place when you start hearing Danger music.